Rishi Sunak is under fresh pressure over his ties to Frank Hester - the party donor embroiled in a race row - amid reports his party received £5m more than previously declared.
The Conservative Party has not denied reports, first published by Tortoise Media, that it received as-yet-undeclared funds from Mr Hester.
Party donations made after December are due to be published in June.
Labour and the Lib Dems have demanded the Tories hand back the money.
Electoral Commission records show Mr Hester has donated more than £10m to the Conservative Party in the past year - including gifting Mr Sunak a £16,000 helicopter journey.
But on Thursday, news website Tortoise Media reported that the Conservatives were "sitting on" an extra £5m cash donation from Mr Hester that had yet to be declared - taking the total to over £15m.
Mr Hester allegedly said veteran MP Diane Abbott made him "want to hate all black women" and should "be shot", sparking days of fierce backlash.
The tech boss apologised for making "rude" comments about the former-Labour MP but said his remarks "had nothing to do with her gender nor colour of skin".
The prime minster called the comments "racist" and "wrong" but argues Mr Hester's apology should be accepted.
During a fiery Prime Minister's Questions session on Wednesday, Mr Sunak rejected repeated requests to hand back Mr Hester's donations.
Responding to the OceanNewsUK's query about a possible further donation from Mr Hester, the Conservatives provided a one-line statement: "Declarable donations will be published by the Electoral Commission in the usual way."
Labour Party chair Anneliese Dodds said: "There is absolutely no excuse for the Conservatives accepting additional money from Frank Hester. They should pay this back before it hits the coffers.
"Rishi Sunak needs to pay back every penny, cut ties with Frank Hester and apologise unequivocally to Diane Abbott."
The Liberal Democrats said if the donation is confirmed "it would show this scandal is even worse than we thought".
Lib Dem MP Wendy Chamberlain said: "People like Mr Hester and his attitudes need to be nowhere near our politics.
"Conservative politicians need to learn that just because someone gives you millions of pounds that does not make the inexcusable, excusable."
