The head of MI6 says Russia broke one of the prime rules of espionage, and won’t be trusted again

The head of MI6 says Russia broke one of the prime rules of espionage and won't be trusted again after it tried to assassinate a former Russian agent despite giving him away in a spy swap.

Alex Younger said British spies had to revise their assumptions about Moscow after Skripal was attacked with a deadly nerve agent, in an operation which Britain has pinned on Russia's GRU spy agency.

Younger is the Chief of the Secret Intelligence Service, more commonly known as MI6, and gave a speech to students at St Andrew's University in Scotland, which was reported by the Financial Times.

In the speech, Younger said the UK had partly trusted Russian President Vladimir Putin when Russia pardoned Skripal in 2010 in return for its own agents.

Younger said that he and his agents assumed that Moscow's spy swap "had meaning" and would be honored, but that they revised their opinion in light of the Skripal attack.

He said, according to the Financial Times: "Mr Skripal came to the UK in an American-brokered exchange, having been pardoned by the president of Russia and, to the extent we assumed that had meaning, that is not an assumption that we will make again."

Skripal was part of an ambitious spy swap deal with the US in 2010 when four Russian agents who had betrayed their country were released by the Kremlin in exchange for 10 Russian spies in the US.

The UK accuses Russia of being behind the attack on Skripal in March 2018, a charge the Kremlin denies.

Novichok, the nerve agent used in the poisoning, has been traced to Russia, and the two men accused by the UK of attempting to assassinate Skripal have been identified by Investigative journalism site Bellingcat as GRU officers.

Read More: The UK government is accusing Russia of waging a secret cyberwar across the globe

Spy swaps are an understanding between the West and Russia that dates back to the of the Cold War.

Jonathan Eyal, international director at think-tank the Royal United Services Institute (RUSI), told Business Insider in March that the safety of the spies is typically put at the forefront of the exchange.

"Spying agencies try to maintain a gentleman's agreement that these people are beyond retribution," he said.

The goal is typically to have them go smoothly so more spy swaps can be done in the future.

Read More: Putin trashes poisoned ex-spy Skripal: 'He's simply a scumbag'

Professor Anthony Glees, the director of the Centre for Security and Intelligence Studies at the University of Buckingham, told Business Insider that the Russians take spy swaps "very seriously" because of the concern that "no one will ever do a swap with them again" if they break faith.

He said that if Russia had really wanted to kill Skripal, it could have executed him in prison.

So Russia would need believe it had a good reason to attempt to assassinate Skripal on UK soil.

"The idea that they would do it for fun or anything less serious is to be discounted," Eyal said. A state of confrontation

Speaking on Monday, Younger said that Russia was in a "perpetual state of confrontation" with the UK, and warned the Kremlin not to underestimate the UK's determination to fight attempts to interfere with its way of life.

"The conclusion [Russia] have arrived at is they should apply their capabilities across the whole spectrum to . . . our institutions and our partnerships," Younger said.

"Our intention is for the Russian state to conclude that whatever benefits it thinks it is accruing from this activity, they are not worth the risk."