The policing of conflicts of interest when former ministers take jobs with private firms is best left to the media, a sleaze watchdog has said.
Lord Mayhew's committee vets ex-ministers who want outside jobs
Lord Mayhew's Committee on Business Appointments vets ex-ministers and civil servants who want to work in industry after they quit government.
But the committee does not check whether they go on to abuse their position once they have been cleared.
Lord Mayhew said media scrutiny was the "best available" method of doing that.
The peer rejected calls for former ministers and civil servants to be banned from taking jobs in their specialist areas for life to avoid potential conflicts of interest.
But Lord MacLennan, who is a member of Lord Mayhew's committee, suggested that ex-ministers who abused their position should face criminal action, in line with the system that operated in the US.
If whistleblowers reported breaches then "the normal forces of the criminal justice system should come into play," said the Lib Dem peer.
Many former ministers from Conservative as well as Labour governments have topped up their parliamentary salaries by taking highly paid jobs in industry when they return to the backbenches.
Lord Mayhew's committee can prevent them from taking lobbying jobs for up to two years if they think there is a risk of impropriety - that they will try to influence government policy or help firms win contracts.
But the peer told the Commons public administration committee bans of that length were "very rare" and it was more common for a 12-month restriction to be imposed.
Asked what happened when a former minister ignored a ban on taking up a particular job, Lord Mayhew said it had never occurred.
But Labour MP Paul Flynn said the system was not suited to the modern world and "stronger" sanctions were needed to restore trust in politics which he said was at an "all-time low".
He called for all public servants to be banned for life from taking up jobs in their own field so that decisions they made in government would not be influenced by the "expectation of retirement riches".
Lord Mayhew said such a ban was "possible" but added that his committee had "no policy in these matters".
He told Mr Flynn that if his committee suspected ministers had been making decisions "with a view to moving into a nice billet in, let us say, a defence supplier, well, then that has to be taken into account and is taken into account by us".
But it was the Conservative peer's personal view that banning them for life would be a step too far as it would "exclude from valuable service in the public interest certain capabilities and experience that ministers have".
Mr Flynn then asked what would happen if an ex-minister who took a job with a private firm and began abusing his or her position as an MP by tabling parliamentary motions in that area.
Lord Mayhew replied: "I think it's very difficult for my committee, in fact it's impossible for my committee, to police the enforcement, the application of the restrictions we impose."
Labour MP Julie Morgan asked Lord Mayhew if effectively leaving the policing of conflicts of interests to the media had "implications for public trust" in politics.
The peer replied that MPs being seen to breach the guidelines would also damage trust and he was "not aware" of any method of monitoring the system other than through media scrutiny.
Ian McCartney is to be quizzed by the committee over his second job
"It certainly can't be policed by us - but to some extent a free media can bring to light breaches of a condition of that character and it does so," he added.
He said leaving the policing of the system to the media was the "best way available, because I can not think of another one at the moment".
The public administration committee was told that only one minister and two "crown servants" had ever been advised not to take a job due to potential conflict of interest - and had always complied with the request.
Lord Mayhew said the prime minister of the day could also overrule the committee, although he said this had only happened once, in 2003, when Tony Blair allowed Air Chief Marshall Sir John Day to become a military adviser to BAE Systems after just three months rather than a year.
The former prime minister said Sir John's appointment was "in the wider national interest".
Mr Blair has taken up a number of highly paid private sector jobs himself since standing down as an MP, including a $5m (£2.5m)-a-year role as an adviser to JP Morgan bank.
Other former Labour ministers, such as Ian McCartney, Richard Caborn, Patricia Hewitt, John Reid, David Blunkett and Alan Milburn, have accepted advisory roles while continuing to sit in Parliament.
Mr McCartney - who is paid £113,000 a year by US nuclear giant Flour - and Mr Caborn, who was appointed as an adviser to construction consortium Amec, but whose fee has yet to be agreed - will both be questioned by the public administration committee in the next few weeks as part of its investigation into lobbying.
The two MPs stress that they are employed as advisers rather than lobbyists and Mr McCartney has written to the committee to say he does not receive any "personal benefit" from his contract with Fluor.
The Fluor cash is paid to a company called Aim & Aim, of which the only directors are the former trade and industry minister and his wife Ann.
Part of the money is used to fund a parliamentary researcher.