diff --git a/registration/templates/registration/payment_form.html b/registration/templates/registration/payment_form.html
index f865187..33c1557 100644
--- a/registration/templates/registration/payment_form.html
+++ b/registration/templates/registration/payment_form.html
@@ -24,25 +24,27 @@
             </p>
 
             <p>
-                {% if payment.grouped %}
-                    {% blocktrans trimmed %}
-                        You want finally that each member pays its own registration? Then click on the button:
-                    {% endblocktrans %}
-                    <div class="text-center">
-                        <a href="{% url 'registration:update_payment_group_mode' pk=payment.pk %}" class="btn btn-warning">
-                            <i class="fas fa-user"></i> {% trans "Back to single payments" %}
-                        </a>
-                    </div>
-                {% else %}
-                    {% blocktrans trimmed %}
-                        You want to pay for the registrations of all members of your team,
-                        or your school will pay for all registrations? Then click on the button:
-                    {% endblocktrans %}
-                    <div class="text-center">
-                        <a href="{% url 'registration:update_payment_group_mode' pk=payment.pk %}" class="btn btn-warning">
-                            <i class="fas fa-users"></i> {% trans "Group the payments of my team" %}
-                        </a>
-                    </div>
+                {% if can_group %}
+                    {% if payment.grouped %}
+                        {% blocktrans trimmed %}
+                            You want finally that each member pays its own registration? Then click on the button:
+                        {% endblocktrans %}
+                        <div class="text-center">
+                            <a href="{% url 'registration:update_payment_group_mode' pk=payment.pk %}" class="btn btn-warning">
+                                <i class="fas fa-user"></i> {% trans "Back to single payments" %}
+                            </a>
+                        </div>
+                    {% else %}
+                        {% blocktrans trimmed %}
+                            You want to pay for the registrations of all members of your team,
+                            or your school will pay for all registrations? Then click on the button:
+                        {% endblocktrans %}
+                        <div class="text-center">
+                            <a href="{% url 'registration:update_payment_group_mode' pk=payment.pk %}" class="btn btn-warning">
+                                <i class="fas fa-users"></i> {% trans "Group the payments of my team" %}
+                            </a>
+                        </div>
+                    {% endif %}
                 {% endif %}
             </p>
 
diff --git a/registration/views.py b/registration/views.py
index 490a187..08e08c6 100644
--- a/registration/views.py
+++ b/registration/views.py
@@ -459,6 +459,9 @@ class PaymentUpdateView(LoginRequiredMixin, UpdateView):
     def get_context_data(self, **kwargs):
         context = super().get_context_data()
         context['title'] = _("Update payment")
+        # Grouping is only possible if there isn't any validated payment in the team
+        context['can_group'] = all(p.valid is False for reg in self.object.team.students.all()
+                                   for p in reg.payments.filter(valid=self.object.valid).all())
         context['bank_transfer_form'] = PaymentForm(payment_type='bank_transfer',
                                                     data=self.request.POST or None,
                                                     instance=self.object)
@@ -489,11 +492,18 @@ class PaymentUpdateGroupView(LoginRequiredMixin, DetailView):
     model = Payment
 
     def dispatch(self, request, *args, **kwargs):
+        payment = self.get_object()
+
         if not self.request.user.is_authenticated or \
                 not self.request.user.registration.is_admin \
                 and (self.request.user.registration not in self.get_object().registrations.all()
-                     or self.get_object().valid is not False):
+                     or payment.valid is not False):
             return self.handle_no_permission()
+
+        if any(p.valid is not False for reg in payment.team.students.all()
+               for p in reg.payments.filter(valid=payment.valid).all()):
+            raise PermissionDenied(_("Since one payment is already validated, or pending validation, "
+                                     "grouping is not possible."))
         return super().dispatch(request, *args, **kwargs)
 
     def get(self, request, *args, **kwargs):