Senior Tory backbencher John Redwood has urged the Government to press for a renegotiation of the UK's relationship with the EU now.
He wrote on his blog:
"The Government is wrong to say we cannot make a move now owing to the problems of the eurozone.
"We need to make the move now for just that reason. The UK needs to be free of any financial liability from the euro troubles, and free of the extra government the EU will now seek to impose to try to 'save' the euro.
"The Government should see what it can negotiate and then put it to the vote. The referendum question should simply be: 'Do you want to stay in the EU based on the new deal on offer?"'
Former defence secretary Liam Fox has said that the UK should leave the European Union unless there is a rebalancing of their relationship.
Foreign Secretary Hague said there would be a "very, very powerful" case for a referendum if Europe moved towards a more federal system.