US Attorney General William Barr sent a memo on Monday in which he wrote that since voting had concluded, it was now "imperative that the American people can "trust" that "the outcomes accurately reflect the will of the voters".

Barr wrote that federal prosecutors could investigate alleged irregularities in the presidential election "if there are clear and apparently-credible allegations of irregularities that, if true, could potentially impact the outcome of a federal election in an individual State".

Mr Barr said prosecutors should only look into "substantial allegations" of irregularities, and that "specious, speculative, fanciful or far-fetched claims" should be ignored.

The attorney general acknowledged that individual states had the primary responsibility for the conduct of elections, but said the justice department had "an obligation to ensure that federal elections are conducted in such a way that the American people can have full confidence in their electoral process and their government".

However, critics have said launching the investigation is undermining the system, as there has been no evidence of widespread wrongdoing.