"I didn’t listen to my staff... and then I nearly fell asleep on stage,” he said.

Mr Biden, 81, last returned from travel on 15 June, nearly two weeks ahead of the 27 June debate.

The president's remarks come amid intra-party panic ahead of November's election over his mental fitness, and after a congressman from Texas became the first sitting Democratic lawmaker to call for him to step aside following the debate.

"I am hopeful that he will make the painful and difficult decision to withdraw," Rep Lloyd Doggett said in a statement on Tuesday.

Mr Biden appeared to struggle through some responses during a debate with former President Donald Trump last Thursday.

"It’s not an excuse but an explanation,” he said at a private fundraiser in Virginia on Tuesday evening, referring to his travel.

He also apologised for his performance and said it was "critical" that he win re-election, according to ABC News.

Mr Biden made two separate trips to Europe within two weeks last month.

On 15 June, he appeared at a fundraiser alongside former President Barack Obama after an overnight trip from Italy. He returned to Washington DC the following day.

White House officials have previously said he was battling a cold on the day of the debate.

The president did not mention any illness in his remarks on Tuesday. A spokeswoman for the White House said earlier in the day that he was not taking any cold medication during the debate.

Mr Biden also spent six days at Camp David, the presidential retreat outside Washington DC, preparing for his debate against Donald Trump.

The New York Times, citing an unnamed source familiar with Mr Biden's schedule, reported on Tuesday that his days began at 11:00 each morning and that he was given time each day to nap.

The newspaper also reported that he was so exhausted from his travel that his debate preparations were cut short by two days to give him time to rest at his beach house in Delaware.

His spokesman, Andrew Bates, said the president began "working well before" 11:00, after his exercise routine, during his time at Camp David.

His age has been a long-simmering issue this election, with voters in multiple polls saying they think he is too old to be effective.

Mr Biden is currently the Democratic Party's presumptive nominee for the White House.

He has vowed to stay in the race despite the debate performance.

In his Tuesday statement, Rep Doggett, 77, said the debate solidified his decision to urge Mr Biden to step aside.

"Instead of reassuring voters, the President failed to effectively defend his many accomplishments and expose Trump's many lies," said the Democratic representative for Austin, who was sworn in in 1995 and is running for re-election.

He said too much is at stake to risk the president losing to Trump over fears about his age.

"While much of his work has been transformational, he pledged to be transitional," the congressman said of Mr Biden.

"He has the opportunity to encourage a new generation of leaders from whom a nominee can be chosen to unite our country through an open, democratic process."

"My decision to make these strong reservations public is not done lightly nor does it in any way diminish my respect for all that President Biden has achieved," Rep Doggett added.