By Ian Dunt
David Cameron is being subject to fresh pressure from the US over his promise of a referendum on EU membership, ahead of a visit by deputy president Joe Biden next week.
The news comes as new polling evidence suggests the pledge did little to beef up Tory poll ratings, although it has had a modest impact on Labour's lead.
The pressure from the US is unlikely to ease over the years ahead, as Washington reiterates its view that it prefers the UK – which usually shares the US' view on international relations – to maximise its influence by staying in the EU.
"With respect to prime minister Cameron's speech, we obviously support his comments about the important role of the UK in the EU," an aide to the vice-president said.
Another told reporters: ‘It's in our interest for there to be a strong UK in a strong EU."
Biden will stop for talks with Nick Clegg before a working lunch with Cameron and a meeting with the National Security Council.
Cameron's referendum pledge has not had the revolutionary effect on the polls his advisers had hoped.
Labour's lead – usually in the double digits – has fallen to five per cent, but the movement comes from voters returning to the Tories or confirming it made them more likely to vote, rather than any influx of Liberal Democrat or Labour voters.
"The speech, and more importantly the policy it articulated, has made Tories feel better about being Tories. This is not to be sneezed at - but let's not confuse it with having changed anybody's mind," Lord Ashcroft, a top Tory donor and pollster, commented on ConservativeHome.
"In debating the question in the coming years we must remember that there is only so much oxygen in the room.
"Most people do not pay much attention to politics at all; when they do, let's make sure they hear something that changes their view of the Conservative party, not just of Europe."
The pledge of a referendum does not even seem to have silenced Cameron's critics.
Following weekend reports of a possible leadership challenge from Tory MP Adam Afriyie, this morning's newspapers contained rumours of new challenges to the prime minister's authority.
The Daily Mail reported that Tory MPs were planning on calling for George Osborne to be replaced as chancellor if he fails to prevent a triple-dip recession.
There are also rumours critics are prepared to trigger a no-confidence vote in Cameron's leadership if the Tories' poll rating does not improve by summer 2014 – a year before the general election.