Boris Johnson has been accused of pushing racial stereotypes over a newly-unearthed column written during his time as editor of the Spectator in which he said young people had “an almost Nigerian interest in money”.

The prime minister has been under pressure in recent months over a series of comments he made during his career as a columnist, writer and editor, including referring to black people as “piccaninnies” with “watermelon smiles” and arguing Islam has caused the Muslim world to be “literally centuries behind” the west.

In another column which has come to light, Johnson blamed single mothers for “producing a generation of ill-raised, ignorant, aggressive and illegitimate children”, saying that social housing was an enticement for them to become pregnant.

The new columns emerged after the chancellor, Sajid Javid, refused seven times to say whether he would use the terms “bank robber” or “letterbox” to describe Muslim women who wear a burqa, as Johnson did last year.

The Conservative party leader wrote in a diary piece for the Independent on Sunday in October 1999 that Tony Blair had made people feel good about getting rich.

He added: “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.”

Weyman Bennett, the co-convenor of Stand Up to Racism, described Johnson’s words as “deeply racist and offensive”. He told the Guardian: “This can be added to the ‘piccaninnies’ and ‘watermelon smiles’, the abject refusal to properly apologise and change his behaviour around racism.

“Boris Johnson is unfit to be a prime minister that represents the entire United Kingdom. He had demonstrated this by lying and falsely representing black, Asian and different communities inside this country.

“This is deeply offensive and unforgivable and should not be ignored and he should be held to account.”

In the column on single mothers, published in the Spectator in 1995, Johnson wrote that “uppity and irresponsible women” had a “natural desire to have babies” and that ways must be found to ensure they married.

Arguing that social housing available to single mothers was “an enticement”, Johnson said “no government – and certainly no Labour government – will have the courage to make the cuts in the safety net of the viciousness required to provide anything like such a deterrent”.

Labour said the comments followed a pattern of sexist and misogynistic words and behaviour by Johnson. Shami Chakrabarti, the shadow attorney general, said: “These unearthed comments further reveal Boris Johnson’s contempt for women and families, as he hypocritically attacks what he appallingly describes as ‘illegitimate’ children.”

In the past 24 hours both major parties have been criticised for their handling of race issues. The Muslim Council of Britain accused the Conservative party of “denial, dismissal and deceit” over the issue of Islamophobia on Tuesday, saying the party had a “blind spot for this type of racism”. On Wednesday Johnson apologised for Islamophobia within the Conservative party.

The Labour leader Jeremy Corbyn has also struggled to move on from an intervention by the chief rabbi, Ephraim Mirvis, who said the party had not done enough to tackle antisemitism.

• This article was amended on 28 November 2019. The original version suggested Johnson accused the Labour government in 1995 of lacking the courage to cut social housing available to single mothers. He actually said “no government – and certainly no Labour government” would have the courage to make such a cut. This has been corrected.