TWENTY-FIVE Tory MPs today lay down a red line on defence by vowing to vote down any fresh cuts to landing forces.

In a letter to the new Defence Secretary, the backbenchers say they are “not prepared to see” any more reductions in the size of the Royal Marines or the Royal Navy’s fleet of amphibious assault ships.

3 Tory MP Johnny Mercer is leading efforts to block new defence cuts Credit: SWNS:South West News Service

The letter – seen by The Sun – instead calls for extra funding to match the recent hike in the threat from ISIS terror attacks and Russia.

The backbenchers’ powerful intervention is a major blow for Theresa May and the Chancellor’s plans.

As the government’s Commons majority is just 13, the group is more than big enough to enforce their will in a vote in Parliament.

It will force Downing Street and the Treasury to rethink plans for the MoD to make up a £2billion-a-year black hole itself.

3 25 MPs have written to the new Defence Secretary Gavin Williamson Credit: PA:Press Association

Packages of cuts drawn up to meet the budget crisis - brought on in part by the pound’s big fall - include selling the Navy’s only two beach landing ships, HMS Albion and HMS Bulwark, and slashing 1,000 Royal Marines.

The bombshell letter was drawn up by ex-Army officer Johnny Mercer, a member of the Commons Defence Committee.

Mr Mercer said last night: “We have drawn up a red line on the strength and capability of our armed forces.

“At some stage you have to stand up for what you believe in, and we have reached that point now. I’m pleased so many of my colleagues agree.”

The MPs’ stand follows The Sun’s revelation on Saturday that new Defence Secretary Gavin Williamson has also decided to fight the bitter new round of defence cuts.

They come in a ‘capability review’ being drawn up by National Security Advisor Mark Sedwill to shift resources from war fighting equipment to intelligence and cyber.

Britain would be left powerless to protect overseas territories like the Falklands or intervene during a humanitarian crisis without the landing ships and enough marines, the MPs warn.

3 Conservatives are concerned that Britain's landing forces are being cut too far Credit: AFP - Getty

The backbenchers write: “We wish to draw your attention to the potential negative impact that any reduction in our amphibious capability would have, and the subsequent consequences on this country’s ability to respond to international events.

“It is simply not credible for us as Government MPs, or a proud island nation, to claim that we are on the side of defence but then allow the Government unhindered to reduce one of our most capable and effective assets – the Royal Marines – and our amphibious capabilities, in particular HMS Bulwark and HMS Albion.

“We must make it clear to you that as a cohort of MPs representing a large portion of the Government’s vote, we are not prepared to see the degradation of this nation’s amphibious capabilities any further.”

The MPs also make it clear to Mr Williamson and the PM that any increase in threat to the nation must be matched by cash and resources.

They call on the Government to match its “rhetoric” on protecting the nation with action.

They add: “We very much look forward to seeing the results of a security review conducted as a result of a ‘significant change’ in the threats we now face as a country, met with the commensurate funding and capabilities.

“As countless Prime Ministers and Defence Secretaries before you have said – it is the first duty of Government to protect the nation. We look forward to that rhetoric being matched in deeds over the coming months.”

While all 25 Tories signed the letter to Mr Williamson, only some are prepared to go public with their signatures now, while others are keeping it an internal matter for the time being.