William Barr, the US attorney-general, authorised prosecutors to investigate possible instances of electoral fraud in the presidential poll, a break from past practice that delayed such probes until after an election was settled, writes FT.

The move came as Donald Trump continued to make claims without providing evidence of fraud in the victory of his Democratic rival, Joe Biden.

The president has refused to concede to Biden, instead filing lawsuits that have been received sceptically by the courts. The Trump campaign on Monday filed a new case in Pennsylvania.

Barr issued a memo to US attorneys across the country authorising prosecutors to investigate “clear and apparently-credible allegations of irregularities that, if true, could potentially impact the outcome of a federal election in an individual state”.

He added: “Nothing here should be taken as any indication that the department has concluded that voting irregularities have impacted the outcome of any election”.