A sleaze watchdog has suggested there should be a limit on how much an MP can earn from a second job and on how many hours they can work outside Parliament before questions are asked.

Lord Evans of Weardale, chairman of the Committee on Standards in Public Life, said he would not advocate a hard limit of “10 hours good, 10 and a half hours bad”.

But he told the Commons Committee on Standards on Tuesday that a rough figure could tell MPs that “at least you know if you go over 10 then you’re likely to be questioned pretty hard – or 20, or 40, or whatever it is – about whether this is reasonable in your circumstances”.

It comes with MPs embroiled in a sleaze scandal over cash being earned outside being a politician.

Lord Evans said there was a fine line because there were good reasons for MPs to have outside roles.

But he added: “Some indication as to what reasonable… or how reasonable should be interpreted or (is) likely to be interpreted is helpful.

“If your job does take you away from your role, not in recess but at other times, for weeks at a time, then I think it certainly raises questions as to whether you are putting the interests of your constituents first or whether you are putting your personal financial interests first.”

He said his committee was not saying MPs should not be able to earn money outside the Commons, but he added: “There is a reasonable expectation on a public servant who is paid a salary on a full-time basis, that that should be their principal activity on behalf of the people who have elected them.”

Lord Evans said it would be “very convenient to say either you’re not going to have any external employment or to say you can do 10 hours and you can earn £20,000”, but that would be too difficult.

Instead, he said there may be “ways of trying to outline where the boundary of reasonable is”, and added: “We suggest that possibly having a rebuttable assumption that, beyond a certain amount of money or beyond a certain number of hours, it is likely to be judged to be unreasonable, might be helpful, not just to those adjudicating, but also to MPs themselves.

“So that it gives a signal to say, look, if you are making £15,000 a year, then it is highly unlikely that this will be judged to be unreasonable.

“If you are making a million pounds a year, then that looks unreasonable on the face of it, and you’re going to have to have (an) extremely strong case as to why, in your particular circumstance, it is not unreasonable to be earning a million pounds a year from external interests, because most people would view that as unreasonable.

“So it’s a pretty clear indication as to where the boundary might lie, but accepting that there may be special cases which you would then have to defend.”

Lord Evans warned there would be a “price to pay” politically if high standards in public life were not maintained.