Three Labour former cabinet ministers face a sleaze investigation after being filmed offering to exploit their government connections for money.

Stephen Byers, Patricia Hewitt and Geoff Hoon were caught by an undercover reporter posing as a US company executive recruiting MPs for lobbying work. Mr Byers, a former Trade and Industry Secretary, was recorded describing himself as a “sort of cab for hire” with rates of up to £5,000 a day.

John Lyon, the parliamentary commissioner, will today be asked to examine possible breaches of rules governing MPs' conduct. The former ministers, who are all stepping down at the election, could all also be investigated for breaking the ministerial code. They all strongly deny any wrong-doing or that they have breached any rules.

The Conservatives will today table parliamentary questions into claims by Mr Byers that he successfully influenced two current cabinet ministers over policy. David Cameron, the Tory leader, described the allegations as shocking.

In an attempt to limit the fall-out from the “cash for access” allegations, Labour was forced to rush forward the announcement of plans for a crackdown on lobbying by former ministers.

According to Channel 4's Dispatches, which worked on the joint investigation with The Sunday Times, Mr Byers told an undercover reporter he had secured secret deals with ministers, could get confidential information from Number 10 and was able to help firms involved in price-fixing get round the law. It said Mr Byers boasted he had struck a secret deal with Lord Adonis, the Transport Secretary, to terminate a rail franchise contract for a client, National Express.

He also claimed to have helped to persuade Lord Mandelson, the Business Secretary, to amend regulations on food labelled on behalf of Tesco. But the day after he was filmed, Mr Byers retracted his claims, insisting he had “never lobbied ministers on behalf of commercial interests” and had exaggerated his influence. Lord Mandelson and Lord Adonis denied Mr Byers' assertions.

Channel 4 said Ms Hewitt, a former Health Secretary, claimed to have obtained a seat on a government advisory group for a client paying her £3,000 a day. She said she “completely rejected” the report. Mr Hoon, a former Defence Secretary and Chief Whip, was reported to have wanted a £3,000-a-day fee for work that would allow him to turn his political knowledge and contacts “into something that frankly makes money”. He said later: “At no stage did I offer, nor would I attempt to, sell confidential or privileged information arising from my time in government.”