LONDON — The former head of Britain’s foreign intelligence service, John Sawers, warned on Friday that the country’s security would be severely compromised if it left the European Union in the spring, and called on voters to reverse the “strategic mistake” in a second referendum.
Mr. Sawers made the comments in a speech in Salisbury, the city where a team of Russian military intelligence officers traveled this spring to attack a former spy, Sergei V. Skripal. Not mincing words, he said that Russia’s president, Vladimir V. Putin, now feels free to “treat Britain with contempt” because, in the age of Brexit and a fraying NATO alliance, it is isolated from its Western allies.
Russia poisoned another Russian, Alexander V. Litvinenko, in London in 2006. Mr. Sawers said it would not have staged another attack had it not perceived Britain as weakened.
“I don’t believe Russia would have used a nerve agent on the streets of an American or German city,” Mr. Sawers said. “The consequences would have been too great, the operation would not have been authorized. But Russia was willing to treat Britain with contempt.”