Chancellor Alistair Darling appeared at a Labour fundraising event hosted by a bank involved in the takeover bid for Northern Rock, MPs have heard.
Mr Darling attended an event hosted by Deutsche Bank
In the Commons, Conservative MP Stewart Jackson questioned the "propriety" of Mr Darling's attendance at the Deutsche Bank function in the City this week.
He also asked whether there was a "possible conflict of interest".
Commons leader Harriet Harman said she was "sure" there was not. Deutsche Bank said it had only supplied the venue.
The Bank of England has lent Northern Rock some £25bn of taxpayers' money after the bank was forced to seek emergency funding in the aftermath of this summer's credit crunch.
During business questions in the Commons, Mr Jackson asked Ms Harman: "Will you give consideration to the propriety or otherwise of the appearance this week of the Chancellor of the Exchequer at a Labour fundraising event hosted by Deutsche Bank, which is a party to the consortium hoping to take over Northern Rock?
"Do you think there is a possible conflict of interest in that particular issue?"
Ms Harman responded: "I am sure there is no conflict of interest."
Speaking later outside the Commons, Mr Jackson said he had put down a parliamentary question asking the Cabinet secretary whether Mr Darling had sought advice about the appropriateness of attending the event.
"It was not one in the normal course of his duties as Chancellor of the Exchequer. It was specifically a fundraiser for Labour," he added.
Mr Jackson continued: "Mr Darling will have a view on the appropriateness of Deutsche Bank and others taking over Northern Rock.
"Also during an internal and police investigation into party funding it is perhaps not very sensible."
A Deutsche Bank spokeswoman said it had just provided the venue for the meeting.
She added: "London in Business hosted an event at which Alistair Darling was present yesterday.
"The event was held at Deutsche Bank's premises in the City and was attended by London in Business guests.
"As a matter of course, Deutsche Bank makes its premises available on a regular basis to a variety of organisations within the City of London."