A senior Labour MP has led calls for his party to back an in-out referendum on the European Union.
Amid public coalition unrest on the question, Home Affairs Select Committee chairman Keith Vaz urged Labour to pledge a referendum in its next manifesto.
Mr Vaz, a member of the Labour for a Referendum group, spoke out after two other Labour MPs in the group, Kelvin Hopkins and John Cryer, signed a Tory amendment on the issue which the Commons is expected to vote on this week.
Mr Vaz, MP for Leicester East, said: "I believe that it is the democratic right of the people to make that decision for themselves. I support Labour for a Referendum's call for the party to support a referendum in our next manifesto."
Twelve other Labour MPs - Jim Dowd, Ronnie Campbell, Rosie Cooper, Ian Davidson, Natascha Engel, Frank Field, Roger Godsiff, Kate Hoey, John McDonnell, Austin Mitchell, Grahame Morris and Graham Stringer - have also signed up to Labour for a Referendum.
Left-wing commentator Owen Jones, actor Richard Wilson and three Labour council leaders are also members of the group.
The group said it would campaign for the Labour Party to adopt the policy, insisting it was popular with members.
Speaking on Saturday, shadow chancellor Ed Balls insisted the Opposition should not "set our face against" a referendum.
He told Sky News's Murnaghan programme: "I think (a commitment to a referendum) is the wrong thing to do now, but I don't think we should set our face against consulting the British people. I don't think we should say anything which gives the impression we don't understand their concerns.
"But I think if we were to answer the question by saying 'maybe in two years' time', we play the same destabilising political, tactical short-term game... (that) is making David Cameron look like he's not really fit to be Prime Minister at the moment."