"This incident is another example of dangerous and unacceptable behaviour by Russian pilots, towards an unarmed aircraft operating in international airspace. These actions create a serious risk of accidents and potential escalation," Defence Minister John Healey said in the communique.

The incident comes as relations between the two countries remain at a historic low over the Ukraine war and longstanding spying allegations.

The Russian Su-35 and Su-27 jets intercepted the Royal Air Force (RAF) Rivet Joint aircraft last month with the Su-27 at one point flying as close as six metres (19 feet) to the British plane's nose, the statement added.

The surveillance aircraft was taking part in a routine flight in international airspace as part of the UK's work alongside allies to secure NATO's Eastern Flank.

Officials said the encounter was the most serious involving a UK Rivet Joint since 2022. In that case, a Russian aircraft fired a missile near a British plane over the same region.

In the new incident, "one Russian Su-35 aircraft flew close enough to trigger emergency systems on the Rivet Joint, including disabling the autopilot system," the ministry said.

"In another dangerous intercept, a Russian Su-27 aircraft conducted six passes in front of the British aircraft," it added.

- 'Russian aggression' -

Healey said the incident would not "deter the UK's commitment to defend NATO, our allies and our interests from Russian aggression".

The intercepts came days after Healey announced that the Royal Navy had tracked and "seen off" three Russian submarines on an alleged month-long "covert operation" in Atlantic waters "north of the UK" near vital undersea cables and pipelines.

Healey made details of the monitoring operation public on April 9.

He warned Russian President Vladimir Putin that any attempt to damage UK cables and pipelines would have "serious consequences".

The UK monitoring mission involved around 500 personnel and saw UK aircraft fly more than 450 hours while a navy frigate covered several thousand nautical miles.

A defence review last year concluded that Russia poses an "immediate and pressing" threat to the nation.

The British government under successive prime ministers has been a staunch supporter of Ukraine following Moscow's February 2022 invasion.

Defence and foreign ministry officials this week formally complained to the Russian embassy about the air incident, the statement added.