The Tories today launched an extraordinary attack on Ed Miliband, claiming that after stabbing his brother in the back he now wants to do the same to Britain. Defence Secretary Michael Fallon claimed the Labour leader was willing to trade away Britain's nuclear deterrent in order to secure power in a backroom deal with the Scottish Nationalists. Mr Miliband said the Conservative campaign had 'descended into the gutter' and claimed David Cameron 'should be ashamed'. Deeply personal: Defence Secretary Michael Fallon  claimed Ed Miliband is 'so desperate' for power he is willing to weaken UK defences to win support from SNP . He confirmed that a Conservative government would go ahead with four new Trident nuclear missile submarines to replace the existing fleet, at a cost of £100billion. Labour rushed to say it was also backing four submarines, despite Ed Balls last month saying he was examining the idea of cutting it to three. Doubt has been cast on Labour's commitment to the deterrent because Mr Miliband could be forced to rely on the SNP support to form a government. SNP leader Nicola Sturgeon has made clear that scrapping the Trident missile system, which is based in Scotland, would be a red line for her propping up a Labour government in Westminster. Mr Fallon launched a highly personal attack on Mr Miliband, saying it was an issue of trust and leadership. He told BBC Radio 4's Today programme: 'We need to be absolutely clear when we come to this decision in the next parliament. 'We saw in that leadership election just what he would do to get into power. We saw what he did to his own brother. We need to know what he would do. 'We can't be sure what kind of backstairs deal he is likely to do with the SNP ... that is the uncertainty. We have seen what he is prepared to do to get into power.' In a speech today, he said: 'Ed Miliband stabbed his own brother in the back to become Labour leader. Now he is willing to stab the United Kingdom in the back to become prime minister.' Mr Fallon will insist the Tories would 'never' do a deal on the future of Trident in another coalition – and that a man prepared to stab his brother in the back to win the Labour leadership cannot be trusted not to risk the nation's security. '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,' the Defence Secretary will say. 'If he wants the keys to No 10, he must abandon any plans to renew our current Vanguard ballistic missile submarines. 'The SNP's childlike world view would sacrifice the long-term security of the UK. The nuclear deterrent protects all of Britain, and the SNP represents a separatist threat that would dangerously weaken our collective defence. Britain doesn't pay ransoms and the Conservatives would never bow to demands from Scottish separatists. Vow: The attack comes as the Tories pledge to build a £100billion fleet of four new Trident submarines . Mr Miliband brushed off the criticism, insisting he had 'got use to' Conservative attacks. Speaking at an event in London, the Labour leader added: 'The Conservative Party can throw what they like at me, they can make all the personal attacks on me that they want to, but I've got use to it. 'And you know what? I'm resilient. And I'm resilient for one reason alone - because I'm fighting for the British people in this campaign. 'I'm fighting for the kind of country the British people want.' Mr Miliband said that over the first two weeks of the Conservative campaign, it was clear 'they have nothing positive to say about the country'. The Labour leader said: 'I think David Cameron should be ashamed. He's got nothing positive to say about the future of the country, he's got no forward vision for the country. And he sends out his minions, like Michael Fallon, to engage in desperate smears. 'I think decent Conservatives right across our country, decent Conservative MPs, decent Conservative party members, decent people right across Britain, will say `Come on, we're better than this kind of politics'. There is mounting Labour panic at polls suggesting the rampant SNP is on course for a landslide north of the border. Such a political earthquake would be likely to leave the SNP holding the balance of power at Westminster and determining who is in government for the first time in the 300-year history of the Union. Shadow foreign secretary Douglas Alexander told the same programme that the Labour Party had made it 'crystal clear' it supports the renewal of Trident. He said: 'This is desperate stuff from a rattled campaign that spent yesterday defending tax avoidance and is now descending into politics of the gutter. 'This is not up for negotiation with the SNP or with any other party. I don't know how I can say it plainer than that. 'We are absolutely committed to a continuous at-sea deterrent. The Tories are like a wounded, cornered animal at the moment.' SNP leader Nicola Sturgeon has made clear that scrapping the Trident missile system, which is based in Scotland, would be a red line for her propping up a Labour government in Westminster . Diane Abbott, who used Twitter to question the £100billion cost of the weapons system . Labour leader Ed Miliband faces a challenge from dozens of Labour candidates calling for Trident to be scrapped. Among them is Diane Abbott, who used Twitter to question the £100billion cost of the weapons system. 'How exactly does submarine system designed for Cold War era combat new threats from international terrorism?' she asked. 'Renewing Trident could cost £100 billion. Would pay for: 150,000 new nurses; 1.5 million affordable homes; 2 million jobs & much more.' Labour CND lists 52 candidates standing for election on May 7 calling for Trident to be scrapped. They call for 'a Labour Government that delivers on disarmament of the UK's nuclear weapons, as we are bound to under the Non-Proliferation Treaty'. Both Labour and the SNP have ruled out a formal coalition, but left the door open to a looser deal that would see Mr Miliband in power even if he wins fewer seats than the Tories. Earlier this year, Mr Miliband suggested he might back a cheaper Trident replacement than the current plan, estimated at £100billion for four submarines at about £5billion each, and annual running costs of £3billion over the subs' lifetime of about 30 years. Today's Tory pledge puts Labour in a difficult position, since ruling out any compromise on Trident would infuriate the SNP and many voters in Scotland, where support for the system's renewal is lower than in the rest of the UK. Mr Fallon will add today: 'It is a sign of his weakness he has failed to rule out a deal. Voters can only conclude he would be prepared to trade Britain's security just to get his hands on the keys to Downing Street. 'The future of our country's security will be on the ballot paper too. There will be a clear choice between a Conservative government that will put our national security first. Or Ed Miliband, a man so desperate for power he is ready to barter away our nuclear deterrent in a backroom deal with the SNP.' Liberal Democrat defence spokesman Sir Nick Harvey said he did not see any need for Trident to be patrolling the seas 24/7 while the UK has 'no known nuclear adversary'. He told BBC Radio 4's Today programme the nuclear deterrent was 'worth retaining', but said two submarines would provide a 'perfectly meaningful deterrent', although not on a 'continuous basis'. Retaining three submarines would enable the UK to have a 'continuous at-sea deterrence', he added. Natalie Bennett, leader of the Green Party, said: 'This latest round of Tory speculation doesn't conceal the fact that the Labour Party is committed to renewing our multibillion pound nuclear weapons system. 'The truth is that the Labour leadership has made it clear that they will prioritise spending £100billion on a cold war relic - rather than investing in the schools and hospitals that this country so desperately needs. 'Only one party is standing up to Trident across the UK. Green MPs will put pressure on the Government to ensure that a proper debate is had over Trident.'
Defence Secretary Michael Fallon launches personal attack on Miliband .
Claims Labour leader would bow to SNP's demands to scrap Trident .
Fallon says 'SNP's childlike world view would sacrifice UK's security'
Miliband claims the Conservative campaign has 'descended into the gutter'