Nearly 500,000 Russian troops have been killed since the Kremlin launched its invasion of Ukraine four years ago, according to a UK spy chief.

Details on the number of Russian casualties were revealed by Anne Keast-Butler, the director of Britain's Government Communications Headquarters.

In her first public speech, Keast-Butler said new intelligence pointed to half a million deaths, which proved Russian President Vladimir Putin was "going backwards on the battlefield". 

READ MORE: Putin's fears of coup ramp up amid Russia's backslide in Ukraine

The figure is equivalent to the number of people living in the Australian Capital Territory.

The spy chief also accused Russian security services of being behind espionage plots in multiple countries.

"One example is in the grey zone between peace and war… where Russia is scaling up its daily hybrid activity against the UK and Europe, stretching from the seabed to cyberspace – relentlessly targeting critical infrastructure, democratic processes, supply chains and public trust," Keast-Butler said.

The Russian government denies it is involved in conducting espionage against the UK and European countries.

Ukraine and Russia have regularly released estimates of the opposing side's losses, but have been reluctant to put figures on their own.

READ MORE: Russia uses hypersonic Oreshnik missile in mass attack on Kyiv

On February 24, 2022, Russian troops, under the direction of Putin, launched a "special military operation" against their neighbour Ukraine, expecting a swift victory.

But advances by the Russian army, which was hampered by poor logistical supplies and cumbersome command structures, quickly slowed.

The invaders met stubborn resistance from the Ukrainian armed forces which were aided by shipments of advanced weapons from their Western allies.

Putin's hopes for speedy battlefield success and the removal of his Ukrainian counterpart Volodymyr Zelenskyy melted away as the conflict became a bloody war of attrition.

GCHQ is the UK's electronic and cyber intelligence agency. It works alongside the domestic security service MI5 and the foreign intelligence agency MI6.

Keast-Butler, the first woman to head the agency, delivered the GCHQ director's annual lecture speech at the agency's World War II headquarters of Bletchley Park.

It is a famed manor north-west of London where hundreds of mathematicians, cryptographers, crossword puzzlers, chess masters and other experts worked to crack Nazi Germany's supposedly unbreakable secret codes.