The letter states: “Your MP will shortly be called upon to support the Prime Minister‘s withdrawal agreement. As a former chief of the secret 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. Please ensure that your MP does not vote for this bad agreement.”