The Metropolitan Police have launched an investigation into allegations of drug use by Lord Sewel and executed a warrant on an address in central London at 6pm.
In a statement, the Met said no arrests have been made at this stage but confirmed a criminal investigation into allegations of drug-related offences involving a member of the House of Lords was under way
The force said one complaint had been received but added the investigation had already begun when this was raised.
Lord Sewel earlier requested a leave of absence from the House of Lords for the duration of investigations into allegations made in The Sun about him taking drugs while cavorting with prostitutes.
The Metropolitan Police statement said: "The Metropolitan Police Service has today, Monday, July 27, launched a criminal investigation into allegations of drug-related offences involving a member of the House of Lords.
"A warrant under Section 23 of the Misuse of Drugs Act 1971, was today granted at Westminster Magistrates' Court. The warrant was executed at 6pm at an address in central London by officers from the Special Enquiry Team of the Homicide and Major Crime Command (HMCC).
"No arrests have been made at this stage and enquiries are ongoing.
"One complaint in relation to this matter has been received. Our investigation commenced prior to this."
The police probe came as the former Labour minister faced intense pressure to quit the House of Lords altogether.
Prime Minister David Cameron led questions about Lord Sewel's future as a peer following two days of lurid allegations on the front page of The Sun on Sunday and The Sun.
Lord Sewel said he would stay away from Parliament during the investigation, writing to the Clerk of Parliaments requesting a leave of absence - a move that can be reversed in future.
The Parliamentary standards watchdog has also been asked to look into the peer's behaviour, amid claims the allegations could mean he had broken House of Lords rules.
During his leave of absence, Lord Sewel will not be able to claim any allowances and must give three months' notice if he wants to attend the House.
He wrote: "I wish to take leave of absence from the House as soon as it can be arranged.
"I also wish to make clear that in do so I have no intention of returning to the House in any way until the current investigations have been completed, when in the light of their outcome I will review my long term position.
"I believe this is compatible with due process."