THERESA May suffered another blow to her Brexit Plan after the former head of MI6 and a former armed forces chief warned it threatened national security and called on MPs to block it.

In a damning letter to Conservative Association chairmen, Sir Richard Dearlove joined forces with Lord Guthrie of Craigiebank, the former Chief of the Defence Staff, describing the Prime Minister’s proposed deal as a “bad agreement” and denouncing the £39 billion divorce settlement as a “ransom”.

They warned the chairmen: “This Withdrawal Agreement, if not defeated, will threaten the national security of the country in fundamental ways. Please ensure that your MP does not vote for this bad agreement.”

Sir Richard and Scottish peer Lord Guthrie stressed the first duty of the state, above trade, was the security of its citizens.

“The Withdrawal Agreement abrogates this fundamental contract and would place control of aspects of our national security in foreign hands,” they declared.

The former security chiefs argued that, if implemented, the PM’s plan would “change our national security policy by binding us into new sets of EU-controlled relationships”.

Sir Richard and Lord Guthrie explained: “Buried in the agreement is the offer of a 'new, deep and special relationship' with the EU in defence, security and intelligence, which cuts across the three fundamentals of our national security policy: membership of NATO, our close bilateral defence and intelligence relationship with the USA, and the Five Eyes intelligence alliance.” The latter involves not only the UK and US but also Australia, Canada and New Zealand.

In an appeal to the Tory association chairmen, they added: "Please ensure that your MP votes against this bad agreement and supports a sovereign Brexit on WTO rules, without payment of ransom, for which we now know from an heroic anonymous civil servant, the civil service is, of course, fully prepared."

David Davis, the former Brexit Secretary, tweeted: "Few people can match the expertise and patriotism of Richard Dearlove and Charles Guthrie. People should pay serious attention to their considered letter on why the PM's Withdrawal Agreement needs to be voted down next week."

But the PM’s spokeswoman hit back, saying: "The claim is completely wrong. Nothing in the Withdrawal Agreement or our Political Declaration cuts across Nato, our defence or intelligence relationship with the USA or with the Five Eyes alliance.

"In fact, our deal delivers the broadest security agreement the EU has with any of its partners," she added.