The Conservative Party received a £50,000 donation from a firm linked to convicted billionaire businessman Prakash Hinduja, new figures show.
The Electoral Commission has revealed the Tories gained £225,587 from private donors in the last week of the general election campaign, bringing their total to £1.8m.
But the biggest single donation came from a firm called Westminster Development Services.
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According to Companies House, the property consortium is up to 50% owned by AMC Project Services Limited, which lists Mr Hinduja as its owner.
He was sentenced to jail last month in Switzerland, along with three of his family members, for exploiting domestic workers at their mansion in Geneva.
However, Mr Hinduja and his wife Kamal, along with their son Ajay and his wife Namrata, were cleared by a Swiss criminal court of more serious charges of human trafficking linked to their servants.
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The four family members were sentenced to between four and four-and-a-half years in prison, but their lawyers said they planned to appeal against the decision.
A Conservative Party spokesperson said: “Donations to the Conservative Party are properly and transparently declared to the Electoral Commission, openly published by them, and comply fully with the law.”
Sky News has also contacted Westminster Development Services for comment.
Elsewhere in the figures, the commission revealed Labour received more than double the donations of the Tories during the last week of the campaign between 27 June and 4 July.
The party racked up £465,600 in those final days, and its entire campaign figure dwarfed its rivals, ending at £9.5m.
The largest donor to Labour in the last week was former professional poker player Derek Web, who gave the party £250,000, followed by both the GMB and Fire Brigades Union, which donated £100,000 each.
The Liberal Democrats also received £156,203 as the campaign drew to a close, with £100,000 coming from food business GADF Holdings, while Reform UK clocked up £45,000, with £25,000 donated by businessman Philip Hulme.
Election rules mean registered political parties have to submit four weekly reports setting out donations and loans they receive above £11,180.