The Conservative party is facing questions over its decision to keep more than £15m given by its biggest ever donor, Frank Hester, after former employees made a series of fresh allegations.

Hester is alleged to have referred to a staff member as the “token Muslim”, imitated people of Chinese descent and remarked that one individual was attractive for a black woman, according to former employees who spoke to the Guardian.

More than a dozen former staffers told the Guardian of claims that Hester repeatedly made comments about race or religion in the workplace or on a work trip, including in recent years.

The claims will pile pressure on Rishi Sunak after it was confirmed on Thursday that Hester had given a further £5m to the Tories through his healthcare tech company, the Phoenix Partnership (TPP). It brings the total donated by Hester to more than £15m in a year, equating to more than 40% of the total national spending limit for each party in the general election.

All the allegations were put to Hester. He did not respond to the claims.

In relation to the new claims, a Conservative spokesperson said the party considered the matter “resolved” as Hester had previously apologised for past remarks about Diane Abbott, which were widely condemned as racist and misogynistic.

Earlier this year, Sunak resisted calls to return the £10m Hester and his firm initially donated, with one of his ministers saying the 2019 comments were made “half a decade ago”.

But in the past six months, former TPP staff have spoken to the Guardian about Hester’s alleged workplace behaviour over a number of years.

Two ex-employees described Hester talking about an individual at a hotel in 2021 and saying she was attractive for a black woman. The comment was allegedly made during a work trip in London.

Other former workers claimed Hester would use a Chinese accent and squint his eyes to narrow them in order to imitate people of Chinese heritage while at TPP’s headquarters in Leeds.

“He was going on a trip to China and he would pull his eyes and say, ‘when are we next going to China?’ in front of Chinese colleagues. You felt like you couldn’t say anything,” one former staffer claimed.

Another individual who worked between 2019 and 2020 at TPP, which holds multimillion-pound technology contracts with the NHS and Department of Health, alleged: “I heard Frank on several occasions use a mocking Chinese accent when saying certain phrases or repeating something an Asian colleague had said.

“It felt humiliating to be so obviously othered in front of a large group. This was behaviour I might have expected in a 70s school playground, not from the CEO of a high-value company at the heart of the NHS.”

Another ex-employee said: “I worked at TPP for about a year and on several occasions Frank would imitate a racist caricature of Chinese people, complete with an exaggerated accent, squinted eyes and a toothy grin.”

Five ex-employees also alleged Hester referred repeatedly to one worker as the “token Muslim” for several years up to around 2016, owing to him being one of the only Muslims who worked at TPP at the time.

One claimed: “He’d often talked about someone in the company being like the token Muslim,”, and another former worker alleged: “There was a ‘token Muslim’ … he was the nicest guy in the world. He would have taken it as a joke. Not that that makes it right.”

Three former workers spoke of Hester having asked for “brown” people to go on a work trip to India in late 2013 to early 2014.

“He was going on a trade mission to India. And he came into the room and said he didn’t have enough ‘brown faces’ to take with him. So can all the brown faces please volunteer,” an ex-staffer claimed.

Another former employee alleged that in late 2013 they were assigned by Hester to work in India because of their ethnic background. They said Hester told them they had been picked “because you are a bit … tinged”.

They claimed that in early 2014, Hester said a new colleague had been hired because “he’s black but he looks more Indian. So we all know what project he’ll be going on.” The former employee said they felt that “it was clear that Frank was aligning people to projects based solely on skin colour”.

In March, the Guardian reported that Hester had told colleagues that seeing Abbott, Britain’s longest-standing black MP, on TV made “you want to hate all black women”. He later apologised for making “rude” remarks but denied they were motivated by race or gender, saying he “abhorred racism”.

The revelation led to cross-party calls for Sunak to return £10m in donations from Hester, which had been made public at that time. West Yorkshire police launched an investigation into “racist comments which were allegedly made at a meeting” by Hester, after it received a complaint from Abbott. The force confirmed this week the inquiry was ongoing.

Reports of a further £5m donation emerged later in March. Data released on Thursday confirmed Hester had given a further £5m through TPP in January this year, and that the Conservative party accepted another £150,000 from TPP three days after the Guardian revealed Hester’s remarks about Abbott.

Abbott said news that the Conservatives had accepted another £5m from Hester was “an insult” to her “and all black women”.

The Guardian previously reported that in 2019 Hester called all his “foreign” workers together to defend himself against online claims that he had made racist remarks. During this meeting, he said he abhorred racism and told his team their progress would not be “based on the colour of your skin, your ethnicity, where your parents are from”.

However, he also said: “We take the piss out of the fact that all our Chinese girls sit together in Asian corner.” During the same, crowded meeting at TPP’s Leeds headquarters, he asked if there was “no room for the Indians” and suggested staff climb on a train roof, before adding that he made “a lot of jokes about racism, about our different creeds and cultures. But I just want to assure you that it is just the most abhorrent thing.”

Hester gave his own definition of racism at the meeting, saying: “For me, racism is just a hatred and a fear of the other. For me, it is just exactly the same as homophobia – it’s not limited to the colour of your skin, it is not limited to religion, it can just be the country next door, it can just be the county next door. It can be northerners and southerners, which we have here.”

Equality campaigners criticised Sunak’s failure to return Hester’s donations at the time, saying an “increasingly normalised culture of racism … has been allowed to fester” under the government.

The Operation Black Vote leader, Simon Woolley, as well as the Runnymede Trust and the Muslim Council of Britain were among those to sign a letter saying: “The donation of £10m to the Conservative party by Mr Hester and his company … becomes a matter of ethical concern and raises questions about the integrity and values that the party wishes to uphold.”

In response to the fresh allegations, Anneliese Dodds, the Labour party chair, said it showed Hester had a “historical record” of making unacceptable comments.

“Rishi Sunak had the chance to cut ties with Frank Hester months ago, but instead accepted further donations from a man who is under investigation from the police for racist remarks. He even told the House of Commons he was pleased to receive Frank Hester’s support.

“Sunak must return the money and get a grip on the extreme views which appear to be tolerated within his party.”

The Conservative party did not directly address questions about what due diligence, if any, it carried out into Hester before accepting further donations from him.

A spokesperson said: “Mr Hester has rightly apologised for comments made in the past. Mr Hester has apologised and shown contrition and we consider the matter resolved.

“The Conservative party is funded by membership, fundraising and donations. All reportable donations are properly and transparently declared to the Electoral Commission, published by them, and comply fully with the law. Indeed, such observations can be made about who our donors are, precisely because our donations are transparently published.

“Fundraising is a legitimate part of the democratic process. The alternative is taxpayer funding of political campaigning, which would mean less money for frontline services like schools and hospitals – or being in the pocket of the trade unions, like the Labour party.”

• This article was amended on 12 September 2024. An earlier version contained a chart that had not fully filtered out donations registered by the Electoral Commission, which were not made directly to the Conservative Party and had therefore overstated the value of donations made other than by Frank Hester.