Shadow foreign secretary Douglas Alexander has branded David Cameron's possibility of an EU referendum as a sign of 'weakness'.
"These latest statements about the possibility of an EU referendum reveal more about David Cameron's present weakness than his future plans.
"Ruling out a referendum one day and then ruling it back in again the next looks like a Prime Minister spending more time managing his party than leading the country.
"David Cameron seems to be more concerned with securing headlines about Europe than securing vital reforms within Europe."
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.