The White House fears the other Russian defectors returned to the West in the Sergei Skripal spy swap will be attacked by the Kremlin.

Senior US administration officials said they had “massive concerns” that Russian critics living in America could be targeted after the Salisbury spy poisoning.

Mr Skripal, who was attacked alongside his daughter Yulia last month, was one of four Russian double agents handed back to the West in 2010.

Igor Sutyagin, a nuclear weapons expert, went to Britain while Alexander Zaporozhsky and Gennady Vasilenko, former KGB officers, went to America.

In turn 10 Russians spies including Anna Chapman, the red-haired woman accused of being part of a “sleeper” cell in America, were returned to the Kremlin.

Previously defectors involved in such public spy swaps were considered safe, given the exchange had been formally agreed by the governments involved.

However following the attack on Mr Skripal the White House now has concerns that those involved in the switch and other Russia critics may now be targeted.