At least one of the two Russian Bear bombers that were escorted away from the Cornish coast entered British airspace, a witness has claimed, contrary to the official version of events.

The apparent incursion, the latest in a series by Russian warplanes amid heightened tension between the two countries, prompted David Cameron to say Moscow was trying to make “some sort of point”. But if encroachment into British airspace was confirmed, the UK government would face pressure to respond more forcefully.

The Ministry of Defence denies that the planes entered British airspace. However, Sue Bamford, from Bodmin, said she witnessed at least one of the bombers flying inland, over Cornwall, while she was having a driving lesson on Wednesday afternoon.

“We were in St Eval when we saw a big black plane that looked like a tank. We thought: where’s that going? It was going along [the route of] the A30,” she said. “As we drove on the big black plane came back again. As Claire [Bamford’s driving instructor] took over to drive back we saw a silver plane, which was the Bear bomber. It’s travelling at the bottom of the St Mawgan valley so we can see it’s not out to sea, it’s in the valley. It’s long and thin, it’s got swept-back wings.”

Bamford said that living between RNAS Culdrose and Newquay Cornwall airport, which used to be RAF St Mawgan, she was used to seeing and hearing different types of aircraft, so she was struck by the sight of two unusual planes on the same day.

Describing the silver plane, she said: “It’s just an odd, odd plane, there’s no other jets in the sky, there’s none of the Typhoons, it’s pootling around and we saw it a couple of times.”

She said that after she went home she did not give any more thought to it until Thursday morning, when the news about the bombers started breaking. “My partner says there’s some Russian bomber off the coast of Cornwall. It’s then that I go online and say that’s the plane I saw, holy crap. It wasn’t out to sea, it’s on St Eval where all the radio masts are, I saw that thing over British land.”

Bamford’s driving instructor, Claire Brazil, from St Austell, said: “I am not an expert but they did look out of the ordinary for Cornish airspace. We leant up to have a look, they were definitely inland, not over the coast. Whether what we saw was the Bear, I don’t know, but it was inland.”

The MoD referred the Guardian to its original statement, which said: “The Russian planes were escorted by the RAF until they were out of the UK area of interest. At no time did the Russian military aircraft cross into UK sovereign airspace.”