diff --git a/services/web/app/src/Features/Subscription/SubscriptionUpdater.js b/services/web/app/src/Features/Subscription/SubscriptionUpdater.js
index fae13eec00..6ea5cd8108 100644
--- a/services/web/app/src/Features/Subscription/SubscriptionUpdater.js
+++ b/services/web/app/src/Features/Subscription/SubscriptionUpdater.js
@@ -274,6 +274,27 @@ async function _notifySupportSubscriptionDeletionSkipped(
   await Modules.promises.hooks.fire('sendSupportRequest', data)
 }
 
+async function _notifySubscriptionDowngradeSkipped(
+  subscription,
+  recurlySubscription,
+  subject
+) {
+  const adminUrl = `${Settings.adminUrl + '/admin/user/' + subscription.admin_id}`
+  const groupUrl = `${Settings.adminUrl + '/admin/group/' + subscription._id}`
+  let message = `\n**Recurly account:** <a href="${recurlySubscription?.account?.url}">${recurlySubscription?.account?.url}</a>`
+  message += `\n**Group admin:** <a href="${adminUrl}">${adminUrl}</a>`
+  message += `\n**Group:** <a href="${groupUrl}">${groupUrl}</a>`
+
+  const data = {
+    subject,
+    inbox: 'support',
+    tags: 'Group subscription',
+    message,
+  }
+
+  await Modules.promises.hooks.fire('sendSupportRequest', data)
+}
+
 async function updateSubscriptionFromRecurly(
   recurlySubscription,
   subscription,
@@ -353,6 +374,8 @@ async function updateSubscriptionFromRecurly(
     return
   }
 
+  const currentSubscriptionPlanCode = subscription.planCode
+
   const addOns = recurlySubscription?.subscription_add_ons?.map(addOn => {
     return {
       addOnCode: addOn.add_on_code,
@@ -371,13 +394,65 @@ async function updateSubscriptionFromRecurly(
   }
 
   if (plan.groupPlan) {
+    if (Settings.preventSubscriptionPlanDowngrade) {
+      // We're preventing professional to standard downgrade.
+      // See https://github.com/overleaf/internal/issues/19852
+      const isProfessionalToStandardDowngrade =
+        currentSubscriptionPlanCode.includes('professional') &&
+        plan.planCode.includes('collaborator') &&
+        !plan.planCode.includes('_ibis') &&
+        !plan.planCode.includes('_heron') &&
+        plan.planCode !== 'collaborator-annual_free_trial'
+
+      if (isProfessionalToStandardDowngrade) {
+        try {
+          await _notifySubscriptionDowngradeSkipped(
+            subscription,
+            recurlySubscription,
+            'Skipped professional to standard downgrade'
+          )
+        } catch (e) {
+          logger.warn(
+            { subscriptionId: subscription._id },
+            'unable to send notification to support that  professional to standard downgrade was skipped'
+          )
+        }
+        subscription.planCode = currentSubscriptionPlanCode
+      }
+    }
+
     if (!subscription.groupPlan) {
       subscription.member_ids = subscription.member_ids || []
       subscription.member_ids.push(subscription.admin_id)
     }
 
     subscription.groupPlan = true
-    subscription.membersLimit = plan.membersLimit
+
+    if (Settings.preventSubscriptionPlanDowngrade) {
+      // We're preventing automatically downgrading of group member limit
+      // See https://github.com/overleaf/internal/issues/19852
+      if (
+        !subscription.membersLimit ||
+        subscription.membersLimit < plan.membersLimit
+      ) {
+        subscription.membersLimit = plan.membersLimit
+      } else {
+        try {
+          await _notifySubscriptionDowngradeSkipped(
+            subscription,
+            recurlySubscription,
+            'Skipped group size downgrade'
+          )
+        } catch (e) {
+          logger.warn(
+            { subscriptionId: subscription._id },
+            'unable to send notification to support that group size downgrade was skipped'
+          )
+        }
+      }
+    } else {
+      subscription.membersLimit = plan.membersLimit
+    }
 
     // Some plans allow adding more seats than the base plan provides.
     // This is recorded as a subscription add on.
diff --git a/services/web/test/unit/src/Subscription/SubscriptionUpdaterTests.js b/services/web/test/unit/src/Subscription/SubscriptionUpdaterTests.js
index e0e8d666bf..b9821f46ff 100644
--- a/services/web/test/unit/src/Subscription/SubscriptionUpdaterTests.js
+++ b/services/web/test/unit/src/Subscription/SubscriptionUpdaterTests.js
@@ -431,6 +431,93 @@ describe('SubscriptionUpdater', function () {
       assert.notEqual(this.subscription.groupPlan, true)
     })
 
+    describe('prevent subscription plan downgrade', function () {
+      describe('prevent professional to standard plan downgrade', function () {
+        beforeEach(function () {
+          this.recurlyPlan.groupPlan = true
+          this.recurlyPlan.planCode = 'collaborator'
+          this.recurlySubscription.plan.plan_code = 'collaborator'
+          this.groupSubscription.planCode = 'professional'
+        })
+        it('should not downgrade from professional to standard plan when preventSubscriptionPlanDowngrade=true', async function () {
+          this.Settings.preventSubscriptionPlanDowngrade = true
+          await this.SubscriptionUpdater.promises.updateSubscriptionFromRecurly(
+            this.recurlySubscription,
+            this.groupSubscription,
+            {}
+          )
+          this.groupSubscription.planCode.should.equal('professional')
+
+          const adminUrl = `${this.Settings.adminUrl + '/admin/user/' + this.groupSubscription.admin_id}`
+          const groupUrl = `${this.Settings.adminUrl + '/admin/group/' + this.groupSubscription._id}`
+          let message = `\n**Recurly account:** <a href="${this.recurlySubscription?.account?.url}">${this.recurlySubscription?.account?.url}</a>`
+          message += `\n**Group admin:** <a href="${adminUrl}">${adminUrl}</a>`
+          message += `\n**Group:** <a href="${groupUrl}">${groupUrl}</a>`
+          expect(this.Modules.promises.hooks.fire).to.have.been.calledOnce
+          expect(this.Modules.promises.hooks.fire).to.have.been.calledWith(
+            'sendSupportRequest',
+            {
+              subject: 'Skipped professional to standard downgrade',
+              inbox: 'support',
+              tags: 'Group subscription',
+              message,
+            }
+          )
+        })
+        it('should downgrade from professional to standard plan when preventSubscriptionPlanDowngrade=false', async function () {
+          this.Settings.preventSubscriptionPlanDowngrade = false
+          await this.SubscriptionUpdater.promises.updateSubscriptionFromRecurly(
+            this.recurlySubscription,
+            this.groupSubscription,
+            {}
+          )
+          this.groupSubscription.planCode.should.equal('collaborator')
+        })
+      })
+
+      describe('prevent decreasing group size', function () {
+        beforeEach(function () {
+          this.groupSubscription.membersLimit = 3
+          this.recurlyPlan.groupPlan = true
+          this.recurlyPlan.membersLimit = 2
+        })
+        it('should not reduce member limits when preventSubscriptionPlanDowngrade=true', async function () {
+          this.Settings.preventSubscriptionPlanDowngrade = true
+          await this.SubscriptionUpdater.promises.updateSubscriptionFromRecurly(
+            this.recurlySubscription,
+            this.groupSubscription,
+            {}
+          )
+          this.groupSubscription.membersLimit.should.equal(3)
+
+          const adminUrl = `${this.Settings.adminUrl + '/admin/user/' + this.groupSubscription.admin_id}`
+          const groupUrl = `${this.Settings.adminUrl + '/admin/group/' + this.groupSubscription._id}`
+          let message = `\n**Recurly account:** <a href="${this.recurlySubscription?.account?.url}">${this.recurlySubscription?.account?.url}</a>`
+          message += `\n**Group admin:** <a href="${adminUrl}">${adminUrl}</a>`
+          message += `\n**Group:** <a href="${groupUrl}">${groupUrl}</a>`
+          expect(this.Modules.promises.hooks.fire).to.have.been.calledOnce
+          expect(this.Modules.promises.hooks.fire).to.have.been.calledWith(
+            'sendSupportRequest',
+            {
+              subject: 'Skipped group size downgrade',
+              inbox: 'support',
+              tags: 'Group subscription',
+              message,
+            }
+          )
+        })
+        it('should reduce member limits when preventSubscriptionPlanDowngrade=false', async function () {
+          this.Settings.preventSubscriptionPlanDowngrade = false
+          await this.SubscriptionUpdater.promises.updateSubscriptionFromRecurly(
+            this.recurlySubscription,
+            this.groupSubscription,
+            {}
+          )
+          this.groupSubscription.membersLimit.should.equal(2)
+        })
+      })
+    })
+
     describe('when the plan allows adding more seats', function () {
       beforeEach(function () {
         this.membersLimitAddOn = 'add_on1'