Vladimir Bukovsky, forced into exile in Britain in 1976, said he had survived three attempts on his life

Britain has tightened security to protect “resettled” foreign spies after alleged intelligence failures over the attempted assassination of Sergei Skripal and his daughter Yulia.

The intelligence services, MI5 and MI6, are understood to have reviewed the risk to spies and defectors from “unconventional” threats such as radiological and chemical weapons after the poisoning of Skripal and Yulia with a nerve agent.

The intelligence community has also pointed the finger at Skripal’s former employer — the GRU, Russia’s military intelligence agency — as being responsible for the assassination attempt.

Whitehall officials told The Sunday Times that MI5 and MI6 had been damaged by the poisonings, which raise questions about their ability to keep recruits alive. “But it’s impossible to reduce risk to zero,” a source said.