A crunch Commons vote on Trident will be held before the Holyrood elections next year after Alex Salmond suggested that the outcome could trigger a second independence referendum.

Conservative Government sources said that the so-called ‘maingate’ decision on the controversial issue is expected to be made before voters go to the polls on May 5.

The timing will lead to fears the issue could dominate the campaign.

A Conservative source insisted, however, that the pre-election date was not fuelled by concerns that the result would boost support for another independence vote.

“This is a very serious matter, with important defence and national security implications. That is what is uppermost in people’s minds,” he added.

But the SNP accused David Cameron’s Government of attempting to rush through the defence system’s renewal at huge cost to the taxpayer.

The party’s defence spokesman Brendan O’Hara said: “The UK Government clearly want to rush through the renewal of Trident at a cost of £167 billion.

“What the Tories face now in Scotland is a cross-party consensus against renewal with Labour joining the SNP in rejecting these weapons of mass destruction, as well as the churches, the trade unions and civic Scotland, who have rejected Westminster’s nuclear arsenal being dumped on the Clyde.’’

A row over Trident erupted this week after more than 70 per cent of both grassroots Labour activists and trade union affiliates at the Scottish Labour conference voted to scrap the nuclear deterrent last weekend.

It is a position the party has not endorsed at an election since the 1980s.

Within days, one of Labour’s most senior MSPs, Jackie Baillie, said that her party needed a “reality check” and warned of the impact on workers.

She said 13,000 jobs could be affected and said her stance had the backing of the general secretary of the Unite trade union, Len McCluskey and the employees at Faslane. “I will be arguing, as I have done consistently, for the workers at the base,” said the Dumbarton MSP.

Divisions south of the Border were also laid bare when Diane Abbott, a close ally of leader and anti-nuclear campaigner Jeremy Corbyn, predicted that the party as a whole would eventually follow Scotland’s lead.

Within hours Maria Eagle, the shadow defence secretary, said that Labour’s policy remained in favour of renewing Trident.

The previous Tory-Liberal Democrat Government agreed to delay the vote on whether or not to replace Trident until 2016, during its coalition negotiations five years ago.

Now sources have indicated that the vote is expected to be held in the spring, before the Scottish Parliament poll.

First Minister Nicola Sturgeon has said that a significant change in circumstance would be needed before she could support a second independence referendum.

Earlier this year, Mr Salmond indicated that he thought one such potential spark could be the renewal of Trident.

The Gordon MP said there were “four issues, any one of which and a combination of them most certainly, could provide a change in material circumstances” adding: “Lots of people are opposed to Trident; it’s in the SNP’s DNA.”

The Conservatives are confident that they will win the Trident vote, despite the party’s slender majority in the Commons.

The Tories expect support from a number of Labour MPs as well as MPs from other parties such as the DUP.

However, the bulk of Scotland 59 MPs will vote against renewal, including the SNP group.

Scotland’s lone Labour MP Ian Murray also opposes Trident.

A senior Tory MP has said that by his calculations the cost of replacing Trident had risen to around £167billion, up from earlier estimates of £100bn.

A Labour spokesman said: “Scottish Labour opposes the renewal of Trident. Labour is reviewing our position and the views of Scottish Labour Conference will feed into this review.”