A former White House ethics chief has said Donald Trump's tweeted complaints about how his daughter was "treated so unfairly" by a retailer amount to "an abuse of the office of the presidency".

Norm Eisen, who served in the Barack Obama administration and later became ambassador to the Czech Republic, said Mr Trump's tweets criticising Nordstrom for dropping Ivanka Trump's brand showed why he "and his family needed to step away, needed to make a more definitive break".

The President tweeted on Wednesday: "My daughter Ivanka has been treated so unfairly by @Nordstrom. She is a great person -- always pushing me to do the right thing! Terrible!" The message was later shared on the official @POTUS account.

Prominent lawyer Mr Eisen added: "I think it's an abuse of the office of the presidency. He's putting the bully in the bully pulpit," The Hill reported.

White House press secretary Sean Spicer later doubled down oin the criticism, saying: "When it comes to his family, he’ll be clear about how proud he is and what they’ve accomplished. For someone to take out their concern with his policies on a family member of his is not acceptable and the President has every right as a father to stand up for them."

Last week, it emerged a number of major US fashion retailers said they would no longer sell Ms Trump’s eponymous clothing and accessories line.

First to make the announcement was Seattle-based Nordstrom. The following day, Dallas-based Neiman Marcus also said it was dropping the line.

"We’ve got thousands of brands - more than 2,000 offered on the site alone," said a spokesperson for Nordstrom, which has nearly 350 stores.

"Reviewing their merit and making edits is part of the regular rhythm of our business. Each year we cut about 10 per cent and refresh our assortment with about the same amount. In this case, based on the brand’s performance we’ve decided not to buy it for this season.”