The Prime Minister issued a strongly worded attack on the allegations about MPs agreeing to influence policy in exchange for cash, saying he was "appalled" by the recent revelations.
His comments came before Richard Caborn, former sports Minister and MP for Sheffield Central, was accused of offering his expertise and contacts to help commercial clients for fees of up to 2,500 after he stands down at the election.
Mr Caborn was also apparently filmed saying he could get "access to Ministers" and "information" if he received a peerage.
The Sheffield MP has denied any wrongdoing and said his comments were taken "out of context", giving the "impression of something improper going on".
He said he made it clear he had not agreed to work for the fake firm, saying: "It would be wrong for me to commit to something that I couldn't do".
A letter from the MP's solicitor said his 2,500 rate reflected three days' work.
Former armed forces Minister Adam Ingram has also become embroiled in the scandal after reportedly saying he could use his contacts to help the fake company develop a relationship with Ministers and civil servants. He too denies any wrongdoing, saying no rules were broken.
The row over lobbying by MPs has become the latest scandal to rock the Houses of Parliament with Liberal Democrat leader Nick Clegg calling the current parliament the most corrupt in British history.
Last week former Ministers Geoff Hoon, Stephen Byers
and Patricia Hewitt were suspended from the parliamentary party after being filmed by Channel 4's Dispatches discussing the possibility of working for what they thought was an American lobby firm. Labour backbencher Margaret Moran, who also featured in the programme, was also suspended.
The Prime Minister said there was a "strong case" for MPs not having second jobs. "I think it's important to recognise when people select or elect Members of Parliament their first priority, their first job, their first loyalty has got to be to the needs of their constituents. I've always acted in a way, and would like other MPs to act in a way, that is clear that the first priority is the interests of the constituents."
Mr Brown said a new system was being put in place as a result of mistakes by MPs that would be "open and transparent" for the future.
"There's got to be a register of lobbying interests, there's got to be openness and transparency in the expenses system.
"I was appalled by what I saw – I was appalled by some of things I saw happening that I knew nothing about."
The interview was recorded before yesterday's allegations regarding Mr Ingram and Mr Caborn, which were made in
The Sunday Times following a joint investigation with Dispatches.
Mr Ingram, MP for East Kilbride, said he was paid 1,500 a day or 1,000 a meeting by companies, the newspaper reported, without specifying what the work was.
However, he said he could not help the company meet serving Ministers until after the election because he was not allowed to lobby as an MP.
Both former Ministers are standing down at the election.
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