Scotland Yard has widened its inquiry into the alleged sale of political honours.
As the investigation continues, it appears set to cause major embarrassment for both Labour and the Conservatives.
In a statement, the Metropolitan Police Service said it had originally received claims about Labour's behaviour.
"These allegations are being investigated by the specialist crime directorate," it added. "This is now a cross-party investigation."
It was not made clear which parties are subject to investigation, but the Conservatives, like Labour, have also been funded by secret loans.
While Labour has named those who lent it money prior to the last general election, the Conservatives have so far declined to do so.
Reports suggest it has, however, been contacting its supporters and asking if their names can be released.
Met deputy assistant commissioner John Yates has already asked the Commons public administration committee to delay its investigation into the honours system while the police review is carried out.
He has also said the police inquiry could be widened to consider more general allegations of corruption.
The latest news comes after the Electoral Commission indicated it may yet force the Conservatives to name the people who lent the party millions of pounds prior to the last general election.
In a statement issued on Wednesday, the watchdog said it remained to be convinced that the loans were made on genuinely commercial terms.
If it is decided that the arrangements brought any benefit then the Commission could require the loans to be treated as donations.
That would mean the names of all the donors would have to be published.
The Commission also said it could use its legal powers to force the parties to hand over documents related to the loans if explanations fail to satisfy it.
