David Veness, Assistant Commissioner at Scotland Yard, said the police had yet to decide whether the disclosure of Tory bank account details confirming that Michael Ashcroft had used a Belize bank to pay the Tory party pounds 1m was a criminal matter.
"We are trying to ascertain whether there's any suggestion of a criminal offence," he said.
He added that the two detectives put on the case from Scotland Yard's fraud squad, S06, were "experienced officers who can appreciate a matter of this delicacy".
Mr Veness said it was too early to say whether senior Tories, including William Hague and Michael Ancram, would be questioned by detectives as part of the inquiry.
The Tory party's announcement that it was calling in Scotland Yard to investigate was seen by the Government as an attempt to switch attention from allegations that Mr Ashcroft's donations may have breached Mr Hague's pledge not to take foreign donations.
Margaret Beckett, the Leader of the Commons, yesterday wrote to Mr Hague demanding a retraction of `slurs' that Labour had been engaged in hacking into Tory files.
She quoted the Tory's bankers, at the Royal Bank of Scotland, saying that there was no evidence that its security had been breached.
"The whole episode is fresh evidence of your bad judgement," she wrote.
The Conservatives tried to turn the tables on Labour by alleging that Labour has large overseas donors including Peter Green, Howard Stringer, Frank Lowe, Michael Watts and Robert Earl.Reuse content