A former M16 head and an ex-chief of defence staff have warned in a joint letter that Prime Minister Theresa May’s Brexit deal is a threat to national security, and advised Tory MPs to vote against the deal.

Former M16 chief Sir Richard Dearlove and ex-chief of staff for the Ministry of Defence Lord Guthrie wrote in the letter published on the Sky News website: “As a former Chief of the Intelligence Service, with my colleague Lord Guthrie, who served as Chief of the Defence Staff shortly before I was in charge of MI6, we are taking the unprecedented step of writing to all Conservative Party chairmen to advise and to warn you that this Withdrawal Agreement, if not defeated, will threaten the national security of the country in fundamental ways.

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

One of the key issues with the Brexit deal is that the government would hand over UK forces and intelligence services to the European Union. It will also damage the UK’s relationship with NATO and the US, as well as security and intelligent partners in Commonwealth countries such as Canada, Australia and New Zealand under the Five Eyes alliance, according to the former chiefs.

The latest Brexit national security warning by Dearlove and Guthrie builds on an earlier warning on 29 November 2018 by Sir Richard and Brexiteers including Sir Rocco Forte, Martin Howe QC, Lord Lawson, Lord Trimble and former Royal Marine chief Major General Julian Thompson.

In response to the first letter, a spokesperson for the Prime Minister at the time denied the claims that the deal would threaten national security.

The spokesperson told Reuters:  “Nothing in the Withdrawal Agreement or Political Declaration cuts across NATO, our defence and intelligence relationship with the USA or the Five Eyes alliance (with Canada, Australia, New Zealand and the United States). In fact our deal delivers the broadest security agreement the EU has with any of its partners.”

Mixed cross-party responses

Labour Party shadow minister for Cabinet office Jo Platt has responded to the new letter, thanking Dearlove for highlighting the danger of the Brexit deal.

She told Sky News: “Theresa [May]’s flimsy negotiations with regards to our national security aren’t good enough, it’s something we need concrete reassurances on and we haven’t got that with this deal.

“If there’s anything the Labour Party would have done, it would have been to secure something as serious as our national security.”

Meanwhile, Home Affairs committee member and Labour MP Stephen Doughty, who supports a final people’s vote on Brexit, said that while Dearlove was right to challenge the deal, leaving the EU without any agreement would harm national security even more.

“As members of the EU we have the European Arrest Warrant available to arrest and extradite terrorism and serious crime suspects, and we have full access to the key databases on crime suspects across Europe that are accessed thousands of times every day by British police forces,” he told Sky News.

“As soon as we leave the European Arrest Warrant will be weakened as Germany, at the very least, will be constitutionally bound to opt out of extraditions to Britain and we have no long-term guarantee of access to the security data we rely on.”

Liberal Democrats MP Tom Brake said he was astonished by the idea that leaving the EU would boost security.

He told Sky News: “We’ve already lost access to the Galileo [satellite] programme and police departments around the country have time and again warned about the threat Brexit poses to their ability to stop terrorists. Then again, this is the guy who was head of intelligence when we went into Iraq.”

MPs are expected to conduct the ‘meaningful vote’ on the Brexit deal on 15 January.