A Liberal Democrat cabinet minister has challenged eurosceptic Tory MPs to "come out" and admit they want the UK to leave the EU.

The comments come as Conservative tensions over Europe increase.

At the weekend almost 100 Tory backbenchers signed an open letter to the Prime Minister calling for the Commons to have a veto on EU law.

Mr Alexander said that he agreed with Foreign Secretary William Hague that that "idea is potty". He added: "It is one that is not going to happen, and I think that those backbenchers who keep putting forward these sorts of ideas should just come out and accept that what they really want is for Britain to leave the European Union - because that is the argument that they are actually making."

Senior LibDems are increasingly concerned about Conservative attitudes to Europe.

While some Tory backbenchers admit they want to leave the EU, no matter the outcome of pledged negotiations to the UK's relationship with Europe, others are keen to stay - as long as there are changed terms.

But some LibDems fear the head of steam currently behind the issue will lead to demands that cannot be met.

David Cameron pledged to renegotiate with the EU last when he committed to an in/out EU referendum in 2017.

The Tory leader had hoped that the promise on a vote on Europe would appease his party's eurosceptics.

But many are underwhelmed by the pledge.

To add to Mr Cameron's headaches, one of his own ministers appeared to disagree with him on Europe yesterday. Ken Clarke rejected claims that Britain's EU membership has been responsible for "unacceptable waves of migration".

His comments appeared to contradict Mr Cameron's warning in December that there should be no repeat of the "vast migrations" from countries like Poland.

The UK has also been warned by former Italian prime minister Mario Monti not to distance itself from the EU. Mr Monti told voters it was "pretty cold" outside the organisation.