By politics.co.uk staff
Backbench Tory MPs may try and force a referendum on the Lisbon Treaty.
A new amendment to the treaty is set to be agreed by ministers in June which will need ratification by member states.
Eurosceptic MP Douglas Carswell told the BBC's Politics Show that it was a chance to have a referendum in Britain on the treaty.
He said: "I think this is an opportunity for us to table an amendment to try to trigger a referendum.
"I think we should have a referendum and I think given that we've given an undertaking as a party to hold a referendum if there was any further revision to the European treaties, we need to honour the promises that were made before the election and stick to that and hold that referendum."
The amendment to the treaty would see extra MEPs being allocated, with Germany losing three and Britain gaining one. It was not in place at the Euro elections in June 2009.
If the EU council of ministers agreed it, it would need legislation in the UK and other member states to make it law.