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There may be a "very powerful" case for an EU referendum if member states agree a closer union, says William Hague.

The foreign secretary said the time to decide would come when it was clear how Europe would develop after the eurozone crisis.

Earlier, David Cameron said he would consider a referendum on the UK's EU relationship, when the time was right.

Nigel Farage MEP, leader of UKIP, described Mr Cameron's position as a "vague promise".

Shadow chief secretary to the Treasury Rachel Reeves refused to rule out the possibility of her party holding a referendum once the future shape of Europe became clearer, but she said Mr Cameron's position on Europe was "a shambles".

Mr Hague said the prime minister would set out his position on a referendum in more detail in the autumn.

'Better relationship'

In an article in the <link> <caption>Sunday Telegraph</caption> <url href="http://www.telegraph.co.uk/news/politics/david-cameron/9367479/David-Cameron-We-need-to-be-clear-about-the-best-way-of-getting-what-is-best-for-Britain.html" platform="highweb"/> </link> , Mr Cameron said he was prepared to consider a referendum but an immediate in/out referendum was not what most wanted.

Speaking on 91�ȱ� One's Andrew Marr Show, the foreign secretary backed the prime minister.

"What the prime minister is saying is that the time to decide on a referendum or in a general election on our relationship with Europe is when we know how Europe is going to develop over the coming months and years to the eurozone crisis," he said.

But, Mr Hague added, the government wanted the opportunity to negotiate a "better relationship" between the UK and Brussels, which would include the return of some powers, before asking the British people to vote.

On EU moves towards a closer union - which is expected as countries try to deal with the eurozone crisis - he said: "If it changes in that way, and once we know whether we can get a better relationship with Europe, then that is the time to make the case for a referendum or, if there is a clear division between the parties, to decide in a general election.

"That is the time, not now."

'No terror'

Many Conservative MPs want the government to make a legal commitment to hold a vote on the UK's relationship with the EU in the next Parliament.

John Baron MP, who sent a letter to the prime minister signed by nearly 100 Tory MPs, said he was "saddened" by Mr Cameron's failure to promise a referendum.

Former defence secretary Liam Fox is expected to say in a speech on Monday that "life outside the EU holds no terror".

The Tory MP will say: "I would like to see Britain negotiate a new relationship on the basis that, if we achieved it and our future relationship was economic rather than political, we would advocate acceptance in a referendum of this new dynamic.

"If, on the other hand, others would not accede to our requests for a rebalancing in the light of the response to the euro crisis, then we would recommend rejection and potential departure from the EU."

Tory Eurosceptic Mark Pritchard said a referendum was needed before the next election.

"Once again, when it comes to Europe, it is always 'jam tomorrow'. But tomorrow never comes," he said.

Speaking on 91�ȱ� One's Sunday Politics programme, Mr Farage said Britain was no closer to a referendum on the EU.

"If he thinks by doing that he's shot UKIP's fox and he's buried this issue in the long grass, he's in for another thing."

He said Tory MPs demanding an in-out referendum were "in the wrong party".

Ms Reeves told the 91�ȱ� a vote was not the right priority while the UK was in recession, but would not rule out Labour support for one in the future.

She said: "Once we know what the eurozone looks like, we can have a discussion about whether it is appropriate to have a referendum."

Meanwhile the Liberal Democrats said existing legislation, which provides for a referendum when there is a proposal to transfer sovereignty from the UK to the EU, was "the sensible way to approach the issue of referenda".

"We understand the internal divisions in the Conservative Party that give rise to this sort of debate," the party said.