Donald Trump has called Joe Biden "the worst president in the history of the United States" following the announcement he has pulled out of the race for the next term at the White House.

Mr Trump did not hold back as he made the statement in a brief phone interview with Sky News' US partner network NBC News, as other Republicans called for the current president to step aside now.

"Joe Biden is the worst president in the history of the United States by far," he said. "There has never been a president who has done such damage to our country. From energy independence to letting in millions and millions of illegal immigrants."

He added: "We will fix what he has done."

The announcement from Mr Biden, 81, comes after months of speculation about his age and fitness to serve another four years.

Asked by NBC if he was surprised by the news, Mr Trump said: "He should never have been there in the first place. He should have stayed in his basement."

Mr Trump, the first former president to be criminally convicted after being found guilty following a trial over hush money in May, also called Mr Biden "crooked" in a statement on his Truth Social platform, saying he "was not fit to run for president... certainly not fit to serve - and never was!"

"He only attained the position of President by lies, Fake News, and not leaving his Basement," Mr Trump continued.

"All those around him, including his Doctor and the Media, knew that he wasn't capable of being President, and he wasn't - And now, look what he's done to our Country, with millions of people coming across our Border, totally unchecked and unvetted, many from prisons, mental institutions, and record numbers of terrorists.

"We will suffer greatly because of his presidency, but we will remedy the damage he has done very quickly. MAKE AMERICA GREAT AGAIN!"

Mr Trump is also facing two remaining criminal cases over the Capitol riots in January 2021 and electoral fraud in Georgia.

In a letter addressed to "my fellow Americans", Mr Biden said he had made the decision in "the best interest of my party and the country" and that it had been the "greatest honour" of his life to serve as president.

The development sends the 2024 presidential contest into unchartered territory just four months before the election. Another Democrat will now need to be chosen to face off against Mr Trump, the Republican nominee, in a race that has already been marked by tension, division and an assassination attempt.

Mr Biden has endorsed Kamala Harris as the Democratic nominee, saying: "Democrats - it's time to come together and beat Trump."

Mr Trump said he believes she will be easier to defeat, according to CNN.

'Bring it on': Reaction from Republicans and supporters

JD Vance, Mr Trump's candidate for vice president, called Mr Biden "the worst President in my lifetime", with Ms Harris "right there with him every step of the way", in a statement on X.

"Over the last four years she co-signed Biden's open border and green scam policies that drove up the cost of housing and groceries," he continued. "She owns all of these failures, and she lied for nearly four years about Biden's mental capacity - saddling the nation with a president who can't do the job."

Mr Vance said he and Mr Trump are "ready to save America, whoever's at the top of the Democrat ticket", and added: "Bring it on."

Other Republicans have also reacted to the incumbent president's news - with Elise Stefanik, House Republican Conference chair, and Republican House Speaker Mike Johnson among those calling for him to resign.

Mr Biden plans to serve out the remainder of his term in office, which ends at midday on 20 January 2025.

"If Joe Biden can't run for re-election, he is unable and unfit to serve as president of the United States," Ms Stefanik said. "He must immediately resign."

Mr Johnson echoed her words and said: "November 5 cannot arrive soon enough."

"If Biden doesn't think he's capable of serving as Commander in Chief for another 4 years, what reason do we have to believe he can do the job for the next 4 months? Who is in charge?" said Texas congressman Pat Fallon.

Steve Daines, senator for Montana, released a statement saying he had concerns for the country's national security as he called on Mr Biden to resign.

"If Joe Biden is no longer capable of running for re-election, he is no longer capable of serving as President," he wrote on X. "Being President is the hardest job in the world, and I no longer have confidence that Joe Biden can effectively execute his duties as Commander-in-Chief."

Ron DeSantis, governor of Florida, criticised Ms Harris as he reacted to Mr Biden's announcement, saying she had been "complicit in a massive cover-up to hide and deny the fact" the president "was not capable of discharging the duties of the office".

He added: "She also was the border czar during the worst border crisis in American history. Democrats are just rearranging the deck chairs on the Titanic."

"Democrat party bosses just proved that they have absolutely no respect for their own voters," said House majority leader Steve Scalise, also on X. "After lecturing others about democracy, they just forced Joe Biden off the ticket - trashing the primary choice of 14 million of their own voters."

Mr Trump's son, Donald Trump Jr, also hit out at Ms Harris in a statement on X, saying: "Kamala Harris owns the entire leftwing policy record of Joe Biden. The only difference is that she is even more liberal and less competent than Joe, which is really saying something.

"She was put in charge of the border and we saw the worst invasion of illegals in our history!!!"