Allegations that MP Mark Menzies misused campaign funds have been referred to Lancashire Police.
The force said it was “reviewing the available information” after receiving a letter “detailing concerns around this matter”.
The PA news agency understands that Labour Party chairwoman Anneliese Dodds wrote to Lancashire Police calling for an investigation into allegations about Mr Menzies.
Mr Menzies disputes the allegations and the Tory Party has said it was looking into the claims and took them seriously.
Labour said the Tories “sat on their hands” for more than three months after finding out about the claims, while the Liberal Democrats called for the Prime Minister’s independent adviser on ministers’ interests to investigate the handling of the row.
Mr Menzies’ former campaign manager who allegedly received a late-night phone call from him asking for cash has said she felt “let down” by the party after she raised concerns with Chief Whip Simon Hart.
The sum, which rose to £6,500, was eventually paid by his office manager from her personal bank account and subsequently reimbursed from funds raised from donors in an account named Fylde Westminster Group, it is alleged.
The Times also reported that £14,000 that was given by donors for Tory campaign activities was transferred to Mr Menzies’ personal bank accounts and used for private medical expenses.
Ms Fieldhouse said she became frustrated with the way the Conservative Party handled her complaint after she reported her concerns to Mr Hart in January.
“I work myself into the ground for the party – all they hear is a 78-year-old little old lady.
“The party has let me down.”
Lib Dem leader Sir Ed Davey said Mr Sunak’s independent ethics adviser, Sir Laurie Magnus, should look into the handling of the row.
“And so the Conservatives have really failed. And therefore, I think there is a role and job for the ethics adviser to investigate.”
Labour leader Sir Keir Starmer told broadcasters: “In the last 24 hours we have found out that the Government was told months ago about this, and they seem to have sat on their hands.
“If they thought they could sweep this under the carpet somehow, they were obviously very mistaken and that is why I think there are very serious questions now that need to be answered.
Meanwhile, Mr Sunak declined to say whether Mr Menzies should quit as Fylde MP.
The Prime Minister told reporters after a speech in London on Friday: “It’s right that Mark Menzies has resigned the Conservative whip.
“He’s been suspended from his position as a trade envoy whilst the investigations into those allegations continue.
“For our part, I can’t comment on our ongoing investigation while it’s happening, and he’s no longer a Conservative MP, as I said.”
A Conservative Party spokesman said: “The Conservative Party is investigating allegations made regarding a Member of Parliament. This process is rightfully confidential.
“The party takes all allegations seriously and will always investigate any matters put to them.”
A spokeswoman for Chief Whip Mr Hart said Mr Menzies had “agreed to relinquish the Conservative whip, pending the outcome of an investigation”, meaning he would now sit as an Independent MP.
