General election: Michael Fallon takes on Ed Miliband over Trident

Michael Fallon on Thursday defended his claim that Ed Miliband was willing to "stab the United Kingdom in the back to become prime minister".

Mr Fallon said that Mr Miliband's defeat of his brother David in Labour's 2010 leadership election "showed just what he would do to get into power" and that he would strike any deal with the Scottish National party in the event of a hung parliament to get in Number 10.

The defence secretary's comments come in the wake of his confirmation of Conservative plans to renew Britain's Trident nuclear deterrent by building four submarines, claiming that national security would be threatened by a Labour post-election pact with the SNP.

Nicola Sturgeon, SNP leader, has said that the scrapping of Trident would be a "red line" issue in any talks with Labour. The Trident fleet is based in the Scottish base of Faslane. But a minority Labour government could still win a vote to renew Trident with the backing of Tory MPs.

Earlier, Tim Montgomerie, the founder of the ConservativeHome website, tweeted: "Embarrassing: way too personal from Michael Fallon against Ed Miliband."

Mr Fallon insisted on the BBC Today programme that there was a "question of trust and leadership" and that Labour would be prepared to strike a "grubby deal" with the SNP after next month's general election.

Douglas Alexander, shadow foreign secretary, said the Labour party was committed to renewing Trident and retaining continuous at-sea deterrence: "I can't be clearer than that," he said.

He accepted that experts believed four submarines were needed to achieve this but that Labour would look at whether technological advances in the coming years might allow the task to be performed by fewer boats.

Mr Alexander claimed that Mr Fallon's comments were "embarrassing, desperate stuff" and showed that the Conservative election campaign was in trouble.

The question of Trident renewal divides the Labour party, with many on the left opposed. The Liberal Democrats support the retention of a nuclear military capability, but with a Trident fleet of perhaps two or three submarines and the abandonment of continuous at-sea deterrence.

Vernon Coaker, Labour's shadow defence secretary, said: "As Ed Miliband has already made crystal clear, national security is not a matter for negotiation. We support renewal of Trident along with a renewed focus on multilateral disarmament.

"Labour is committed to maintaining a minimum, credible, independent nuclear deterrent, delivered through a 'continuous at-sea deterrent'. This is not up for negotiation with any party," Mr Coaker said. Labour has said that it will study whether this policy can be achieved by deploying three submarines rather than four, but senior military figures have dismissed this idea as impracticable.

Mr Fallon, in a speech in London on Thursday, will say: "Nicola Sturgeon could not be clearer. She has told Ed Miliband that scrapping Trident - our country's vital nuclear deterrent - is a red line she will not cross. If he wants the keys to Number 10, he must abandon any plans to renew our current Vanguard ballistic missile submarines."

Mr Miliband has ruled out a full coalition with the SNP in a hung parliament but has not ruled out a less formal pact. Ms Sturgeon, for her part, has promised to stop David Cameron returning to Number 10 and has offered to help Mr Miliband gain power.

Meanwhile, Mr Cameron has warned Ms Sturgeon that he will not countenance a second independence referendum, saying the issue had been "settled", possibly for a lifetime.

Ms Sturgeon was booed by some of the studio audience in a televised election debate on Tuesday when she refused to rule out calling for a second referendum in the SNP's Holyrood manifesto next year.

But Mr Cameron told The House magazine that he shared the view of Alex Salmond, the former Scottish first minister, that last year's referendum had settled the issue "for a generation, possibly for a lifetime".

He added: "I'm sticking with that. I think there was a very big debate in Scotland, a very big moment, a very big turnout. But it was pretty decisive, a 10-point margin is pretty decisive."

Scottish voters rejected independence last September by 55 per cent to 45 per cent.

The Westminster parliament would have to agree to a second Scottish referendum, raising the possibility of a clash between Holyrood and the rest of the UK if Ms Sturgeon pressed the case.

South of the border, a recent drop in support for the UK Independence party has been confirmed by opinion polls in 10 Conservative marginal seats in England, with supporters of Nigel Farage's party switching to both Tories and Labour.

The surveys by Lord Ashcroft, the Tory peer, reflect a "poll of polls" in The Sun newspaper, which suggested that Ukip had lost a quarter of its support in four months - falling to 12.2 per cent.

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The latest Ashcroft research found that Ukip's share of the vote in nine of out 10 Tory marginal seats polled had fallen by up to 10 points, boosting support for the Conservatives.

But the polls suggest that Labour had also benefited from the Ukip squeeze and Mr Miliband's party has been further helped by a big decline in the Liberal Democrat vote.

The result is that in the 10 marginal Tory seats polled by Lord Ashcroft, the Conservatives are in the lead in five (Blackpool North and Cleveleys, Gloucester, Kingswood, Loughborough and Pendle) while Labour leads in four (Hove, Morecambe and Lunesdale, Stockton South and Harrow East). Pudsey was tied.

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