Russia mounted the Salisbury nerve agent attack because it believed Britain was "weak and isolated" after the vote to leave the EU, a former MI6 chief has said.
Sir John Sawers said he did not believe the Russians would have used a nerve agent on the streets of a US or German city in the way they are alleged to have done in the attempted assassination of former spy Sergei Skripal.
"They thought they could pick on Britain and bully us because we were looking weak," he told BBC Radio 4's The World at One.
"Actually Theresa May and her Government responded very strongly and forcefully to the Skripal attack but it was only because we were able to carry our European partners, the Americans and others, with us.
"It was through engagement with others that we were able to push back and have a credible response to the Skripal attack."
Following the attack in Salisbury in March, in which Col Skripal and his daughter Yulia survived, the Government blamed the Russian GRU military intelligence agency.
The aftermath saw more than 20 countries expel a total of more than 100 Russian diplomats despite fierce Kremlin denials of any involvement.
There was fresh outrage when Dawn Sturgess and her partner Charlie Rowley became infected with the nerve agent novichok used on the Skripals, after picking up a discarded perfume bottle. Ms Sturgess later died.
Sir John warned in future Britain's diplomatic leverage would be weakened as it heads out of the EU in March 2019.
"Our strength in the world has come from our ability to work with both the United States and our European partners," he said.
"The more influence we have with the European Union, the more weight we have with America and vice versa.
"The Americans are to some extent walking away from their relationship with Europe, the transatlantic relationship, and we are walking away from the European Union. The West is fragmented."
He added: "What we are doing is losing traction both in Washington and in Europe, and that will make Britain less influential on the world stage."