Rishi Sunak continues to resist calls by former prime minister Boris Johnson to supply fighter jets to Ukraine.
Mr Johnson has used a US tour to push for the West to step up military assistance for Kyiv, dismissing arguments against the supply of sophisticated Nato aircraft.
But Downing Street said it would take years to train Ukrainian pilots to fly the RAF’s Typhoon and F-35 planes and said the current Prime Minister was focused on providing support that would be of use now.
At an event hosted by the Atlantic Council think tank, Mr Johnson said: “I hear that an objection to their having sophisticated Western planes to fly is that they won’t know how to use them.
“I have to say I take that argument with a bit of a pinch of salt.”
But in Downing Street, the Prime Minister’s official spokesman set out the practical difficulties in supplying British warplanes to Ukraine.
“We will continue listening to the Ukrainians and consider what is right for the long term,” the spokesman said.
“But it’s helpful to understand the situation, that the fastest training programme for a new pilot is approximately 35 months.
“The current UK fast jet training programme takes five years.”
Mr Sunak’s strategy was to “focus on how we can help Ukraine defend their country and push back this year”, the spokesman added.
Downing Street has previously stressed that Mr Johnson was “acting in his own capacity and not on behalf of the UK Government” during his visit to Washington.
Defence Secretary Ben Wallace refused to rule out sending jets in the future.
He said there was “no magic wand in this horrendous conflict” but “on the question of jets, one thing I’ve learned over the last year is don’t rule anything in, don’t rule anything out”.
But he said training Ukrainian pilots to use UK fighter jets would “take months”, and suggested the priority should be helping train Kyiv’s forces to take back territory.
Speaking in Portsmouth alongside Australian counterparts, Mr Wallace said Russian forces were resorting to First World War-style “human wave” tactics against Ukrainian positions.
“They are resorting to First World War-level style of attacks, with the subsequent casualties to match.”
