Prime Minister Gordon Brown said there is a "strong case" for MPs not having second jobs and he was "appalled" by recent revelations as a fresh row over lobbying emerged.
Two more former Labour ministers were secretly filmed offering their Government connections and expertise for cash, according to the Sunday Times.
Adam Ingram, the former armed forces minister, and Richard Caborn, the former sports minister, denied any rules were broken and said their comments were taken "out of context".
But it follows the suspension of three former Labour ministers from the Parliamentary party last week after similar allegations.
Mr Brown told BBC Scotland's Politics Show: "I think there's a very 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 is being put in place as a result of mistakes by MPs that will be "open and transparent" for the future.
"So all MPs activities have got to be clear," he added.
"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."
