William Barr, the former US attorney general who served under Donald Trump, has said the ex-president is "toast" if allegations he mishandled classified documents are proven to be true.

Mr Barr criticised his former boss and said he had no right to keep the files allegedly found at his home.

Mr Trump will appear in court in Miami on Tuesday to face dozens of charges accusing him of illegally retaining classified information.

He has repeatedly denied wrongdoing.

Speaking to Fox News on Sunday, Mr Barr, who was Mr Trump's attorney general from February 2019 until December 2020, defended the 37-count indictment made public by Special Counsel Jack Smith on Friday.

"I was shocked by the degree of sensitivity of these documents and how many there were... and I think the counts under the Espionage Act that he wilfully retained those documents are solid counts," he said.

"If even half of it is true, then he's toast. It's a very detailed indictment and it's very damning," he added.

The 73-year-old was once one of Mr Trump's staunchest allies, but has been increasingly critical of him since leaving office. Shortly after he gave the interview, Mr Trump described him as a "'disgruntled former employee" and "lazy attorney general who was weak [and] totally ineffective".

Many prominent Republicans have been hesitant to criticise the former president, who is the frontrunner to be the party's presidential candidate in 2024, and have instead targeted the justice department and the broader investigation.

A poll released on Sunday by the BBC's US partner CBS found 76% of likely Republican primary voters were more concerned about the indictment being politically motivated than about the documents posing a national security risk.

The indictment accuses him of keeping hundreds of classified documents, including some about US nuclear secrets and military plans, at his Florida estate Mar-a-Lago after he left office.

It alleged that he lied to investigators and tried to obstruct their investigation into his handling of the documents.

Under the Presidential Records Act, White House records are supposed to go to the National Archives once an administration ends. Regulations require such files to be stored securely.

Legal experts say that the criminal charges against Mr Trump, who is running for president again in 2024, could lead to substantial prison time if he is convicted.

Special counsel Jack Smith, who oversaw the investigation, has denied the charges are politically motivated, saying: "We have one set of laws in this country, and they apply to everyone."

Mr Trump is expected to travel to Miami from his Bedminster, New Jersey, home later on Monday. He will stay in the city overnight before travelling to the federal courthouse for an initial appearance on Tuesday.

The city is braced for supportive protesters to greet him on arrival at court, and the mayor will outline security plans later.

After his hearing, Mr Trump plans to make remarks to the media from his Bedminster golf club on Tuesday evening.