The Lords authorities have promised a "rigorous" inquiry
Police are looking at newspaper allegations that four peers were prepared to change laws in return for cash, it has been confirmed.
Officers from the Metropolitan Police are reviewing material relating to the claims, made in the Sunday Times, following a complaint by the Lib Dems.
But Lady Royall, Leader of the House of Lords, said police had yet to decide whether to launch an investigation.
She has said the allegations, which the four men deny, are "very serious".
The Sunday Times reported that four Labour peers - Lord Moonie, Lord Truscott, Lord Taylor of Blackburn and Lord Snape - had discussed amending legislation in return for a fee, which would be in breach of parliamentary standards.
The four deny breaking any rules.
The Lib Dems alerted the police to the allegations on Monday.
Lady Royall said she had been told officers were looking at material relevant to the issue while attending a meeting of the Metropolitan Police Authority.
In a statement in the House of Lords, she said: "On Monday, the Met Police received a request to consider investigating whether an offence had been committed by certain members of the House.
"The police have now decided to review the relevant material in this matter to assist them in deciding whether it would be appropriate to carry out an investigation."
She added: "I should stress that the police are not investigating this matter at this stage."
A Met spokesman said it would be "reviewing the relevant material to assist a decision whether or not to it will be appropriate to launch an investigation".
A House of Lords committee is to investigate the allegations, which have raised concerns across the political spectrum.
Lady Royall has said the allegations have damaged Parliament and that its members must abide by the highest standards.
The government has said it will act decisively if any wrongdoing is uncovered, introducing new sanctions against members if necessary.
But opposition parties say the allegations show the need for more fundamental reform of the House of Lords.