diff --git a/addons/account/account_invoice.py b/addons/account/account_invoice.py
index f255b6788ee..662b7103358 100644
--- a/addons/account/account_invoice.py
+++ b/addons/account/account_invoice.py
@@ -308,7 +308,7 @@ class account_invoice(osv.osv):
         '''
         Find the partner for which the accounting entries will be created
         '''
-        #if the chosen partner is not a company and has a parent company, use the parent for the journal entries 
+        #if the chosen partner is not a company and has a parent company, use the parent for the journal entries
         #because you want to invoice 'Agrolait, accounting department' but the journal items are for 'Agrolait'
         part = inv.partner_id
         if part.parent_id and not part.is_company:
@@ -419,7 +419,7 @@ class account_invoice(osv.osv):
         try:
             compose_form_id = ir_model_data.get_object_reference(cr, uid, 'mail', 'email_compose_message_wizard_form')[1]
         except ValueError:
-            compose_form_id = False 
+            compose_form_id = False
         ctx = dict(context)
         ctx.update({
             'default_model': 'account.invoice',
@@ -451,11 +451,15 @@ class account_invoice(osv.osv):
             context = {}
         invoices = self.read(cr, uid, ids, ['state','internal_number'], context=context)
         unlink_ids = []
+
         for t in invoices:
-            if t['state'] in ('draft', 'cancel') and t['internal_number']== False:
-                unlink_ids.append(t['id'])
+            if t['state'] not in ('draft', 'cancel'):
+                raise openerp.exceptions.Warning(_('You cannot delete an invoice which is not draft or cancelled. You should refund it instead.'))
+            elif t['internal_number']:
+                raise openerp.exceptions.Warning(_('You cannot delete an invoice after it has been validated (and received a number).  You can set it back to "Draft" state and modify its content, then re-confirm it.'))
             else:
-                raise osv.except_osv(_('Invalid Action!'), _('You can not delete an invoice which is not cancelled. You should refund it instead.'))
+                unlink_ids.append(t['id'])
+
         osv.osv.unlink(self, cr, uid, unlink_ids, context=context)
         return True
 
@@ -540,11 +544,11 @@ class account_invoice(osv.osv):
         return result
 
     def onchange_payment_term_date_invoice(self, cr, uid, ids, payment_term_id, date_invoice):
-        res = {}        
+        res = {}
         if not date_invoice:
             date_invoice = time.strftime('%Y-%m-%d')
         if not payment_term_id:
-            return {'value':{'date_due': date_invoice}} #To make sure the invoice has a due date when no payment term 
+            return {'value':{'date_due': date_invoice}} #To make sure the invoice has a due date when no payment term
         pterm_list = self.pool.get('account.payment.term').compute(cr, uid, payment_term_id, value=1, date_ref=date_invoice)
         if pterm_list:
             pterm_list = [line[0] for line in pterm_list]