Diane Abbott has said the Conservatives are “only paying lip service to fighting racism” after it emerged their controversial donor Frank Hester had given a further £5m to the party before the election.

The donation, made by his company the Phoenix Partnership, brings his total funding to the Tories to more than £20m, cementing his status as their single biggest donor.

Hester was at the centre of a political furore in March after the Guardian revealed he had told colleagues at his IT healthcare company in 2019 that looking at Abbott made you “want to hate all black women” and that she “should be shot”.

The police opened an investigation, Operation Brassminster, in March, which is continuing.

Hester issued a statement apologising for his remarks about Abbott, describing them as “rude”, but said his “criticism had nothing to do with her gender nor colour of skin”. The statement said Hester abhorred racism, “not least because he experienced it as the child of Irish immigrants in the 1970s”.

Despite widespread outrage at the remarks and their eventual condemnation by Rishi Sunak, who called them “wrong” and “racist”, the Conservatives have continually resisted calls from Labour and other parties to return the £20m from Hester and his company.

At the time, and during the election, the party repeatedly refused to reveal whether he had given any more money after the row escalated.

After the further £5m was revealed in Electoral Commission data on Thursday, Abbott said: “Even Rishi Sunak had admitted reluctantly that Hester’s remarks were racist. But they still took his money. It shows that the Tories only pay lip service to fighting racism.”

She told the Guardian: “The fact that none of the leadership candidates will say that they would not take more of his money tells you that this entire slate of candidates is firmly on the right.”

Katie White, the new Labour MP for Leeds North West, where Hester’s firm is based, added: “Electoral Commission data shows that Frank Hester’s millions fuelled the Conservative party’s general election campaign – funding the leaflets arriving at our doors, the targeted ads flooding our social media, and the political advisers downplaying the issue … By accepting and using his money, the Conservative party is, in effect, condoning these deeply troubling comments.”

The fact the latest £5m was donated days before the election meant it did not become public during the six-week contest, when parties have to give regular updates about donations during the campaign.

Other big donations to the Tories in the second quarter of the year include £1.25m from Access Industries, a company founded and owned by the Soviet-born US-British businessman Sir Len Blavatnik.

Hester’s original £10m of donations were given to the Tories in 2023, and a further £5m was accepted in January. Three days after the Guardian’s original story, the Conservatives accepted a further donation of £150,000 from the Phoenix Partnership. That donation had been received on 8 March and was accepted by the party on 14 March.

Hester’s donations to the party since the beginning of 2023 represent 58% of the total national spending limit of £35m for each party in the general election, meaning returning it would have created a significant shortfall.

In April, Richard Holden, the Conservative chair, refused four times when questioned on the BBC’s Politics Live to say whether the party had accepted a further £5m from Hester. This followed reports that Holden had said he was “comfortable about accepting money when people have been clear about their views”.

Holden said: “Mr Hester apologised fully for his comments at the time and I think if people have apologised then we should accept that, when they have clearly made a major contrition. I can’t comment on individual donations. It would be inappropriate.”

• This article was amended on 20 September 2024 to clarify that Sir Leonard Blavatnik is the founder and owner of Access Industries; an earlier version said only that it was “linked” to him.