The outgoing head of MI6 has issued a damning indictment of Russia’s invasion of Ukraine, saying that Vladimir Putin has “bitten off more than he can chew”.

Richard Moore, known within MI6 as C, used his farewell speech in Istanbul to say that while Russia was unlikely to win on the battlefield, his agency was seeking to recruit spies inside Russia and worldwide in order to fight back.

“Putin has sought to convince the world that Russian victory is inevitable, but he lies. He lies to the world. He lies to his people. Perhaps he even lies to himself,” said Moore. “But we should not believe him. Or credit him with strength he does not have.”

Any Russian victories on the battlefield remained incremental, said Moore, and incurred massive costs as the president’s army fell far short of its original aims to swiftly capture all of Ukraine.

The rising costs now included more than a million casualties – a quarter of them “poorly trained troops from Russia’s poorest regions” that were “fed into the meat grinder”, he said.

A former ambassador to Ankara, Moore appeared to choose Istanbul as the place to launch a drive to recruit spies due to the large numbers of Russian visitors to Turkey as well as its proximity to Ukraine.

The Turkish president, Recep Tayyip Erdoğan, has long sought to position his country – a Nato member and the host of slow peace talks that have come to little – as an important interlocutor with Moscow.

The outgoing spy chief also cited other threats to British and global security, notably Iran. Millions of Iranians visit neighbouring Turkey each year, and Moore pointedly added that MI6’s recruitment offer via a new portal on the dark web was open to anyone.

“Our door is always open,” he said. Citing his entreaties to Russian citizens during a previous visit to Prague, he added: “Come spy with us.”

Iran, China and North Korea had all aided Putin’s war in Ukraine, Moore said, allowing Putin to avoid possible internal collapse or an inevitable ceasefire deal.

“In the end, if we hold our nerve, Putin will need to come to terms with the fact that he has a choice to risk an economic and political crisis that threatens his own rule, or make a sensible deal,” he said. “This is a choice he would have had to confront earlier, if not for the outside help he had been receiving.”

Moore used his speech to launch MI6’s secure messaging platform, named Silent Courier, as a way for potential recruits to contact the organisation and offer their services. He presented this as a way to help Britain and its allies repel Russia’s invasion of Ukraine and possibly accelerate the decline of Putin’s rule.

“Not all Russians subscribe to Putinism,” he said. “Some keep their heads down and try to get on with their lives as best they can. Some, like Alexei Navalny, resist openly and die for their beliefs, but others do so secretly – by working for MI6.”

Promising potential Russian recruits security and protection, Moore framed the offer as an opportunity to bring peace to Europe.

“To those men and women in Russia who have truths to share and the courage to share them, I invite you to contact MI6. You will be working to bring peace to our continent, to protect the long term interests and redeem the honour of your country.”