David Cameron has effectively resigned as Prime Minister after declaring an EU referendum a "red line" in any coalition negotiations, a senior Liberal Democrat MP has said.

Tim Farron, the former Lib Dem president who has been tipped as a successor to Nick Clegg as leader, said Mr Cameron had been "foolish" to set out such a big red line, which he said contrasted with the "small number of red lines" set out by his party.

It suggests he would want to block his party from accepting a referendum on the EU as part of a second coalition deal with the Conservatives. It was the first time Mr Cameron revealed a "red line" for coalition negotiations.

Asked during the Question Time leaders' debate which policies he would be prepared to compromise in coalition discussions, the the Prime Minister said: “The British people really do deserve a referendum on whether to stay in a reformed European Union or leave... I will not lead a government that doesn’t honour that pledge. That’s a red line.”

Mr Farron, speaking to The Independent after the debate, appeared delighted with Mr Cameron's new 'red line': "That’s him resigning on Friday. Cameron’s just resigned his leadership."

He added: "We’ve been very, very careful to set out a small number of red lines – less than last time around. Cameron has really foolishly picked one red line that 90 per cent of the electorate – whatever their views on Europe – most people think that it’s about 107th in their list of priorities.

"It's very risky to give him the vote when the only promise he’s made is to plunge the country into economic chaos and undo the good work of the last five years.

"He’s either told us a big, big lie or he’s resigned – we’re in the last 7 days of Cameron’s premiership."

The latest red line to be outlined by Mr Clegg is a commitment to spend an extra £8 billion a year on the NHS by 2020.

This follows three other red lines set out this week. These include pledges to increase education spending in England from £49bn to £55.3bn over the next five years, raising the basic rate income tax threshold to £12,500 and holding a "stability budget" within the first 50 days of a new Parliament.

In pictures: Experts' predictions for the General Election - 26/04/15 Show all 10 1 /10 In pictures: Experts' predictions for the General Election - 26/04/15 In pictures: Experts' predictions for the General Election - 26/04/15 Andrew Hawkins (ComRes) “This is an election of wild promises and political cross-dressing; so no differentiation, no breakthrough, no majority. The most likely outcome is Ed Miliband in No 10 because only he will be in a position to work with enough parties (that is, the SNP) to form a workable government.” Andrew Hawkins In pictures: Experts' predictions for the General Election - 26/04/15 Joe Twyman (YouGov) “As each day passes and the Conservatives fail to increase their vote share, the likelihood of Ed Miliband becoming PM (albeit still with a minority) grows. That’s due to the continued performance of the SNP in Scotland and former Lib Dem voters propping up Labour in England.” In pictures: Experts' predictions for the General Election - 26/04/15 Ben Page (Ipsos MORI) “A better week for Ed Miliband than for David Cameron – personal ratings improving, and the betting markets moved in his favour. The poll numbers have him ahead by a whisker which, combined with the electoral maths, makes him more likely to be able to form some kind of government on the raw numbers right now. However, Cameron will be hoping for a 1992 moment when ultimately voters can’t quite bring themselves to try someone else – and 38 per cent of voters still say they may change their mind.” In pictures: Experts' predictions for the General Election - 26/04/15 Rick Nye (Populus) “The Populus/Hanover Predictor has Ed Miliband ending up as Prime Minister in more than five out of six simulations we run.” In pictures: Experts' predictions for the General Election - 26/04/15 Nick Moon (GfK) “Tories most seats, SNP 45 or more, Ukip fewer than five: minority Labour government.” In pictures: Experts' predictions for the General Election - 26/04/15 Damian Lyons Lowe (Survation) “Ukip’s vote share is prone to be squeezed, benefiting the Conservatives, while the SNP’s onslaught on Labour continues. Based on three UK/Scotland polls, I am upgrading the Conservatives and the SNP, and downgrading Labour. Both the Lib Dems on 30 seats and Ukip on 5 to 10 remain unchanged.” In pictures: Experts' predictions for the General Election - 26/04/15 Michelle Harrison (TNS) “Still no change, but Nicola Sturgeon goes from strength to strength in Scotland. For me, who the biggest party is will hinge on the crucial marginals. But what ultimately wins – safety first, for the Tories, or Labour’s ‘get out the vote’ ground war?” In pictures: Experts' predictions for the General Election - 26/04/15 James Endersby (Opinium Research) “Our poll today shows a one-point Tory lead, statistically a tie. It’s going to be incredibly tight. Labour and the Tories would do well to be negotiating with the smaller parties behind the scenes. In the likely event of a hung parliament, we’ve been testing various combinations of coalition. Labour teaming up with the Lib Dems would be more popular than with the SNP, which in turn would be more popular than a Tory/UKIP scenario.” In pictures: Experts' predictions for the General Election - 26/04/15 Martin Boon (ICM) “Not much going on really, and no reason to change my mind. Tories by a couple of points. In pictures: Experts' predictions for the General Election - 26/04/15 Lord Ashcroft (Lord Ashcroft Polls) He refuses to make predictions. “My polls are snapshots, not predictions.” Rex

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