Washington DC - President Donald Trump mocked Joe Biden Wednesday by putting a picture of a so-called "autopen," instead of his likeness, in a new gallery of presidential portraits in the White House.

Republican Trump has alleged, without evidence, that Biden's use of the automated technology to sign pardons and other documents was part of a cover-up of the Democrat's cognitive decline.

Now he has doubled down on the allegation by putting a picture of Biden's signature and the device, in place of his portrait, in a new "Walk of Fame" located in the colonnade that runs along the White House Rose Garden.

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"The Presidential Walk of Fame has arrived on the West Wing Colonnade. Wait for it..." Trump's Communications Advisor Margo Martin posted on X, along with a video of the new display.

The camera pans along a line of black and white, gold-framed presidential portraits, hanging along the colonnade in chronological order, before it reaches the picture of the autopen representing Biden.

The White House separately posted a photo of Trump himself – whose portraits as the 45th and 47th US president are on either side of Biden's – looking at the new addition.

The Biden autopen portrait will be visible by guests attending a dinner later Wednesday in the Rose Garden, where Trump has recently replaced the grass with a patio.

Trump had previewed the move in an interview with the conservative Daily Caller earlier this month, saying: "We put up a picture of the autopen."

The 79-year-old almost obsessively bashes his now 82-year-old predecessor, seeking to blame him for a host of ills, including inflation and the wars in Ukraine and Gaza.

The Biden autopen portrait is, meanwhile, the brashest step yet in billionaire property developer Trump's revamp of the more than 200-year-old presidential residence in Washington.

Trump notably has covered the Oval Office with gold decorations, installed huge flagpoles, and is now building a giant ballroom.

He has also moved a painting of Democrat Barack Obama from its original position and, breaking with precedent, hung several paintings of himself in the White House.