Jeremy Corbyn is facing renewed questions over his fitness to be prime minister after a day of confusion over Labour's defence policy.
The party leader came under fire from senior military figures after he appeared to call into question Labour's commitment to maintaining Britain's Trident nuclear deterrent.
There was further controversy after he suggested he would be reluctant to authorise a drone strike on the leader of the Islamic State terror group and could suspend RAF attacks against the extremists in Syria and Iraq.
General Lord Richards of Herstmonceux, a former chief of defence staff, told The Daily Telegraph: "Jeremy Corbyn, unlike many of his distinguished predecessors in the Labour Party from Clement Attlee through Denis Healey and beyond, has demonstrated why he should not be trusted with the ultimate responsibility of government – that of the nation's defence and security."
The Conservatives – who said he had left Labour's defence policy in "chaos" - signalled that they intended to make his suitability for high office a key issue in the general election campaign.
The row helped deflect attention from the Tories' own difficulties at the weekend over their policies on pensions and taxation.
Theresa May still faces questions over whether she will continue with David Cameron's commitments not to put up to income tax, VAT or national insurance and to maintain the pensions "triple lock".
Mr Corbyn's problems came during an interview on BBC1's The Andrew Marr Show in which he said he would order an immediate strategic defence review looking at "all aspects" of defence policy if he was prime minister after June 8.
The Labour leader – a lifelong opponent of nuclear weapons - went on to say there was a "discussion" going on within the party as to whether the existing commitment to maintain Trident would be in the election manifesto.
With the party's position threatening to unravel, a spokesman finally issued a statement, declaring: "The decision to renew Trident has been taken and Labour supports that."
The issue of the deterrent has proved highly divisive for Labour, with Mr Corbyn finally abandoning his attempt to persuade the party to support unilateral nuclear disarmament at last year's annual conference following opposition from the trade unions.
The Labour leader will attempt to put his difficulties behind him when he makes his first campaign visit to Scotland, following Mrs May's decision last week to call a snap election.
In an address to the Scottish TUC, he will reaffirm his commitment to repealing Conservative trade union legislation while fighting to win votes in "every corner of these isles".
"While the timing of the election was unexpected, the choice is clear and the stakes are high. Let no one be in any doubt - we are in this election to win it and we will fight for every seat in every corner of these isles," he will say.
"The choice facing the country is clear. It's the people versus the powerful.
"Labour will challenge the rigged system that is holding our country back. And just like trade unions, we will stand for the many not the few."