Joe Biden is not senile and has been on “good form” at the Nato summit, Sir Keir Starmer has said, as concerns grow about the US President’s mental fitness.

The Prime Minister defended Mr Biden’s leadership credentials amid questions about his cognitive health, which have intensified since criticism of his performance in a TV debate against Donald Trump.

The president, 81, is seeking re-election in November despite discontent within the Democrat ranks about his suitability for a second term in office.

Sir Keir Starmer met US President Joe Biden at the White House (Stefan Rousseau/PA)

Sir Keir, who met the president for their first bilateral talks at the White House on Wednesday as Nato leaders gathered in Washington to discuss the challenges facing the bloc, said Mr Biden had been “across all the detail”.

The Prime Minister told broadcasters their one-to-one discussions had happened “at pace” and Mr Biden seemed “on really good form”.

“We were billed for 45 minutes, we went on for the best part of an hour,” he said.

“He was absolutely across all the detail. We were going at pace through a number of issues.”

Asked whether the president was senile, Sir Keir told the BBC: “No… he’s shown incredible leadership.

“If there’s one thing that came out of the council session yesterday it was a clear understanding by everybody here that we’re faced with more threats now than we’ve faced for many years, and that we need the resolve of Nato.

“President Biden has led through some of the most challenging issues, actually, that we’re facing globally.”

The president on Monday stood firm against suggestions he should drop his candidacy and appealed for an end to the row over how the party moves forward in a sharply worded letter to Democrats.

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Legislators and fundraisers including Hollywood actor George Clooney have since joined calls for Mr Biden to step aside, after he appeared flustered and stumbled over his words in the presidential debate with Mr Trump last month.

Sir Keir’s latest comments come after the two leaders spoke on the sidelines of the Nato summit as the Prime Minister seeks to strengthen what he called the “very special relationship” between the UK and US.

On Wednesday the Prime Minister, who has proposed forcing peers to retire from the House of Lords at 80, had sidestepped a suggestion that indicated Mr Biden was too old, instead saying he was motivated by reducing the size of the upper chamber.

He told reporters accompanying him on his first international trip as prime minister: “In terms of the age in the House of Lords, the simple fact is that our House of Lords is massive. That is the primary driver of the retirement at 80.”

Their meeting came as Sir Keir prepared to urge Nato allies to support Ukraine for “as long as it takes”, as he confirmed the UK’s own £3 billion a year military aid for Kyiv will continue.

Sir Keir told the president: “The special relationship is so important. It’s forged in difficult circumstances, endured for so long, and stronger now than ever.”

Meanwhile Mr Biden earlier appeared to back Sir Keir Starmer’s push for the UK to forge closer defence ties with Europe, as he called Britain the “transatlantic knot” binding the two continents together.

Sir Keir wants an ambitious new UK-EU security pact to strengthen co-operation and seeks closer work on defence with key allies such as France and Germany.

The president said: “I kind of see you guys as the knot tying the transatlantic alliance together, the closer you are with Europe. We know where you are, you know where we are.”

The Washington summit has also cemented Ukraine’s “irreversible” progress towards Nato membership, with a joint statement by the allies.

But because of Nato’s Article V, which commits to mutual defence, the allies will not allow Ukraine to join while it is still at war, as that would trigger an immediate conflict with Russia.

Nato allies are stepping up commitments to Ukraine while they await the results of the presidential election in the US, where Republican candidate Mr Trump has threatened to reduce American support.