THE Defence Secretary will announce today the replacement of Trident is already underway – before a crucial vote on renewal of the controversial nuclear deterrent has taken place.

The declaration will come just days after Nicola Sturgeon pledged to make the weapons system a key issue in the forthcoming Scottish election.

Conservative cabinet minister Michael Fallon is expected to reveal an extra £640m for the programme.

The statement will take the amount already being spent on Trident's successor to almost £4bn.

But the declaration that debate around Trident is effectively over is likely to trigger an explosive response from the SNP and other opponents of nuclear weapons.

The party has previously accused UK ministers of trying to pre-empt a Commons vote on the issue.

The SNP warns ministers want to introduce a new generation of nuclear weapons by the back door, by spending more and more money on Trident's development before MPs get a say.

Speaking ahead of a visit to Scotland later this week, Mr Fallon said: “The most important point to make about the Trident submarine programme is that it is happening – it is already underway.”

He added that it was “extraordinary” for opposition politicians to “almost deny that it is happening”.

A total of 57 of Scotland's 59 MPs oppose Trident renewal.

At a demonstration in London at the weekend Ms Sturgeon accused Tory ministers of being "intent on basing another generation of nuclear weapons in Scottish waters."

She told the crowd: "There have been reports that the Tories have been fretting about when to hold their Parliamentary vote on Trident renewal.

"They were worried that opponents might make it an election issue over the next couple of months.

"Well, we have a message for them - you bet we're going to make it an election issue."

Part of the £640m money will be used for missile compartments to be built by Babcock in Rosyth.

Mr Fallon confirmed that the vote would not be on the so-called ‘maingate’ decision on whether or not to replace Trident.

Instead, MPs will be asked to support the principle that the deterrent should be replaced with a like-for-like system.

Mr Fallon said that offering MPs a maingate vote could “reduce” the Ministry of Defence’s bargaining power with defence companies.

He added: “Parliament authorises the money separately. There is no pressing finical reason at all (to hold this vote n the near future).

"But I think it is right for continuing public support of the programme to have a parliamentary vote in favour of it.”

He also predicted that, with the support of most Labour MPs, the Conservative Government would win the vote “handsomely”.

Tory ministers have been accused of delaying the vote simply to prolong Labour's agony over the issue.

Labour leader Jeremy Corbyn, a long-term opponent of nuclear weapons, is currently attempting to overturn party policy in favour of Trident renewal.

Mr Fallon said that it was “unlikely” a vote would be held before the end of the current parliamentary session, but added that he hoped it would be called in the “early summer”.

The Defence Secretary said that previous investments had helped to safeguard up to 100 jobs at Rosyth.

He also predicted that than 13 frigates would be built on the Clyde in coming years.

He said ministers had guaranteed to build 8 frigates designed to protect the Navy's new aircraft carriers.

But, he added, "we’re also going to design a lighter general purpose frigate and overall I hope the total will be more than 13 to replace the current fleet.”

In a speech to the Scottish Conservative conference on Friday Mr Fallon is also expected to name RAF Lossiemouth, in Moray, as the preferred base for one of the RAF’s two new Typhoon squadrons.

While in Edinburgh he will also visit a software company which develops mine detection technology.