A top-secret British spy plane was tracked using a £2.99 mobile phone app as it flew on a daring mission to eavesdrop on Vladimir Putin’s air defences.

The £650 million RAF aircraft, called Rivet Joint, could be seen at 27,000ft as it tried to gather intelligence about a heavily defended Russian base on the Baltic Sea.

A minute-by-minute record of the highly classified mission to study the Kaliningrad naval air base was watched by hundreds of thousands of people on the internet, and details were shared on Twitter.

A top-secret British spy plane was tracked using a £2.99 mobile phone app as it flew on a daring mission to eavesdrop on Vladimir Putin’s air defences (Stock photo)

Radar pictures on social media suggested the RAF jet flew to within 60 miles of the Russian base at 478 knots, accompanied by American spy planes and F-35A stealth jets.

The joint UK-US operation, launched last Tuesday, came at a time of high tension between President Putin and Nato and just days after British troops began manoeuvres in Estonia intended to prevent a Russian invasion.

Last night, former Air Commodore Graham Pitchfork warned that websites such as Flightradar24.com and Planefinder.net had made it almost impossible for the RAF to maintain operational secrecy.

He said: ‘There’s not much these clever geeks can’t watch and share with the world. The speed of transfer of the information and the ease with which it is shared is remarkable.

‘I find it slightly odd that these people want to track our military spy planes everywhere, but there you go.

‘It is right that the issue is highlighted but I fear we’re going to have to live with it.’ The 146-ton Rivet Joint and her 27-strong crew took off from RAF Waddington in Lincolnshire on Tuesday morning. The 136ft-long aircraft was later seen crossing the North Sea and flying over Denmark.

Open Secret: The £650 million aircraft was tracked near a naval air base

Plane-spotters logged the aircraft’s speed, direction of travel and flight number as she approached the Russian coast.

Security experts said the plan was for Rivet Joint to fly behind the F-35As and respond electronically to any attempt by the Russians to track the F-35As.

Justin Bronk of the Royal United Services Institute think tank explained: ‘Rivet Joint takes a very sensitive reading of Russian air defences, such as their search radars. It is very useful for the UK and US to know the sorts of patterns that the Russians’ frequency-agile radars operate with.

‘The UK and US aircraft were most likely trying to provoke a response, because usually the Russians wouldn’t broadcast their highest end radar waveforms for Britain and the US to collect and analyse.’

The Mail on Sunday has previously revealed how, using the Flightradar24 app and mobile phones, terrorists could have tracked Prince William’s air ambulance helicopter

Mr Bronk said Rivet Joint’s position was revealed by its transponders which can only be turned off in a war zone.

‘European airspace is extremely crowded so there is a limit to what it can do silently,’ he added. Transponders record an aircraft’s position using GPS satellites, then broadcast the information to ground stations monitored by the likes of Flightradar24.

Defence sources said the RAF is mandated by the International Civil Aviation Organisation to ensure that in peacetime its aircraft comply with safety procedures, including the use of transponders.

The Mail on Sunday has previously revealed how, using the Flightradar24 app and mobile phones, terrorists could have tracked Prince William’s air ambulance helicopter.

Last night, a Ministry of Defence spokesman said: ‘The RAF has a wide variety of cutting-edge intelligence-gathering capabilities, which may be used in association with our allies, to obtain vital intelligence across the globe. As a matter of policy we do not comment on speculation on operational or intelligence matters.’