Mr Johnson, who used to work as a journalist, wrote a piece in 1999 about how young people could be attracted back to the Conservative party.

This was two years into the first government of Labour Prime Minister Tony Blair when Conservative popularity was at a low ebb.

"We had to recognise that Blair had found a way of making people feel good about getting rich," Mr Johnson wrote.

"All the young people I know - ie. those under 30 - are just as avaricious as we flinty Thatcherite yuppies of the 1980s; in fact,they have an almost Nigerian interest in money and gadgets of all kinds."

Anti-racism campaigner Weyman Bennett told the UK's Guardian newspaper that Mr Johnson’s words were “deeply racist and offensive”.

The BBC has asked the Conservative Party to comment.

In the past, Mr Johnson has apologised for a 2002 column on Mr Blair's trips abroad in which he wrote:

"It is said that the Queen has come to love the Commonwealth, partly because it supplies her with regular cheering crowds of flag-waving piccaninnies...

"They say he is shortly off to the Congo. No doubt the AK47s will fall silent, and the pangas will stop their hacking of human flesh, and the tribal warriors will all break out in watermelon smiles…"

Mr Johnson's comments about Nigerians have surfaced as the row continues about anti-Semitism in the main opposition Labour party. Party leader Jeremy Corbyn has come under fire for not saying he is sorry about it in a BBC interview.