Stephen Byers, the former transport secretary, today insisted he had complied with the MPs' code of conduct after a group of Labour MPs were targeted in an elaborate sting operation in which journalists set up a bogus lobbying company and offered to pay them in return for political influence.

Byers, an arch Blairite, was filmed describing himself as a "bit like a sort of cab for hire" and offering to trade Westminster contacts for £3,000 to £5,000 a day.

Amid condemnation from senior cabinet ministers, the Labour party tried to limit the damage by saying some MPs were "mortified" by how stupid they had been, and promising a crackdown on lobbying by former ministers, rushing forward a manifesto pledge for tighter regulation and monitoring.

Others who were targeted in the undercover operation included former cabinet ministers Geoff Hoon and Patricia Hewitt. Margaret Moran, Labour MP for Luton South, was also involved.

The chancellor, Alistair Darling, said it was ridiculous that the MPs had been caught out in the sting. He told BBC1's Andrew Marr show: "The best answer when you get a call like that is to put the receiver back down again. It's obvious.

"There are rules about serving MPs ‑ we've said that we're going to have to, I think, get a statutory-backed code of contact to deal with former ministers. But really, what on earth did they think they were doing? And equally for a company, you don't need a lobbyist. If you've got something to say, go directly to the government department and make your case. It's just ridiculous."

The foreign secretary, David Miliband, said he was appalled by today's revelations. He told Sky News: "There is absolutely no room for anyone to trade on their ministerial office. People come into politics ‑ whether Labour, Tory or Lib Dem – because of what they want to do for the country. And I believe that's true for MPs across all parties, I don't think this is a partisan point.

"Anything which sullies that reputation or gets in the way of that public service is completely inimical. I think it's right that we have tightened up the rules already ... but the Labour manifesto is going to say more about the need for a statutory register of the lobbying industry, because there is absolutely no room for the sort of innuendo or promises that seem to have been floated in this case."

In the sting, 20 MPs were invited to attend meetings to discuss joining an advisory board and 10 turned up. The meetings were mainly held at offices in London's St James's Square. An undercover Sunday Times journalist asked them how the company could go about influencing policy and how it could improve its chances of winning a government contract.

In a statement issued this afternoon, Byers said: "I am confident that any investigation from the standards commissioner will confirm that I have always fully complied with the MPs' code of conduct. I have never lobbied ministers on behalf of commercial organisations and have always fully disclosed my outside interests."

Byers had told the bogus lobbyists he had saved hundreds of millions of pounds for National Express through his contact with Lord Adonis, the transport minister, and had influenced food labelling proposals for Tesco after phoning Lord Mandelson, the business secretary. The MP said that his friendship with Mandelson was one of his "trump cards".

However, the next day he sent an email to the meeting's organisers saying he had "overstated" the part he had played in trying to secure changes to the way in which the government deals with issues. "This means that I have not spoken to Andrew Adonis … or Peter Mandelson about the matters I mentioned," he wrote.

Byers issued a statement last night saying that at an informal meeting about a potential job opportunity he had made some exaggerated claims.

"Having reflected on my comments I knew that I should immediately put the record straight. I did so the following morning by making it clear that I have never lobbied ministers on behalf of commercial interests. I later withdrew my name for consideration. I have always fully disclosed my outside interests," he said. Byers described the set-up as a "massive deception".

A statement issued by Adonis's office today said: "There is no truth whatsoever in the suggestion that Stephen Byers came to any arrangement with Andrew Adonis on any matter relating to National Express. It is equally untrue to suggest that National Express were allowed by the government to avoid any of their rail contract obligations."

A spokesman for Tesco said: "We did not speak to Mr Byers on food labelling, regulation or indeed any other issue. These claims are completely fictitious and Mr Byers has acknowledged this to us."

The operation features in a Dispatches programme to be aired tomorrow on Channel 4. The journalists set up a lobbying company known as Anderson Perry Associates, supposedly based in the US. Its website described it as a "bespoke consultancy that helps organisations and individuals maximise and exceed expectation". It claimed to have 120 clients in Europe, the Middle East and the US, operating in the health and defence industries.

Another of those filmed was the ex-health secretary Patricia Hewitt, who said she "completely rejected" the suggestion that she helped obtain a key seat on a government advisory group for a client paying her £3,000 a day.

She stressed that the role she had been discussing would only have been taken up after she stepped down as an MP at the imminent election, and insisted there was "nothing unusual or improper in the business appointments that I have taken up since leaving government".

Hoon said he had not offered to lobby government, nor had he broken any rules. "I was written to by what seemed to be a reputable American company. They had a website, and addresses in both the United States and St James's Square, London," he said in a statement.

"I was asked to visit their offices to have what they described as 'an informal chat'. This took place after the announcement of my decision to leave parliament before the next election. I have not been a minister since June 2009.

"In the course of what I assumed to be a private conversation, I was asked whether I might be interested in joining the advisory board of a UK company that they were thinking of establishing.

"There was a discussion about my qualifications and experience for such a position. I made clear in the course of the conversation that I would not lobby government and that I was not interested in work of that kind.

"At no stage did I offer, nor would I attempt to sell confidential or privileged information arising from my time in government. I did not break any parliamentary or ministerial rules in the course of the conversation."

Hoon told the undercover reporter he was interested in "translating my knowledge and contacts about the international scene into something that frankly makes money".

But he went on: "I do not want to be seen to leave politics and go back as some sort of lobbyist. I am quite happy with strategic advice ... and in the right circumstance I don't mind leading a delegation in to see a minister, but that's not what I want to spend my life doing."

The exposé is likely to thrust the issue of standards back to the heart of the election campaign as party leaders battle to show they will clean up parliament.

David Cameron promised tougher controls last month, warning that secret corporate lobbying was the "next big scandal waiting to happen" in Westminster after expenses. He wants ministers to be forced to wait two years after leaving government before being allowed to lobby – twice the present limit – and harsher penalties for those who breach the rules.

Tony Wright, the Labour MP who chairs the select committee which called for a statutory register, welcomed the government's change of policy.

"Our report both made the case for effective regulation of lobbying and showed how it could be done. We need to know who is lobbying whom about what, and that needs a statutory register. This is becoming normal in many other countries but we said at the time that it would probably take a lobbying scandal before we got action here. Now that we have had a scandal, let's hope effective action of the kind we proposed will follow."