A complaint against Peter Robinson that has been referred to the Metropolitan Police has been dismissed by his spokesman as a "childish prank".
The Met has been asked to take on an investigation into the complaint made by Belfast businessman Peter Curistan.
The commissioner of the London force received the request in a letter from the Police Service of Northern Ireland.
The complaint relates to remarks the first minister made in the House of Commons in 2006 while he was an MP.
Mr Curistan's lawyers allege there is a case to answer in respect of "misfeasance in public office", which they claim is not covered by parliamentary privilege.
They claim Mr Robinson, the Democratic Unionist Party (DUP) leader, has falsely linked the developer of Belfast's Odyssey complex to "IRA dirty money" and that has had "enormous consequences" on his business life.
The PSNI has told Mr Curistan's legal team the request was made to the Met because "the alleged offences took place in London".
What is misfeasance in public office?
Misfeasance in public office is where a public body, an individual in public office or a public servant acts unlawfully, knows that they are acting unlawfully and does so knowing that his or her actions are likely to cause loss or harm to another person.
A statement from the DUP said: "The Metropolitan Police are likely to be even more dismissive of this childish prank than the PSNI were.
"It is unsurprising that the BBC gives it publicity."
Responding to the development, Mr Curistan's solicitor, Joe McVeigh, said: "It may be helpful to have outside officers look at the case.
"However, most of the consequences of the remarks occurred in Northern Ireland.
"We will be asking the PSNI why they have assessed this as being a matter for the Met."
Previously, Mr Robinson issued a statement through the DUP after being informed of the initial complaint.
He said: "The police may have to waste their time with this but I do not."