Ed Miliband has called for a cap on how much MPs can earn doing second jobs.
The Labour leader raised the controversial topic in his speech on party finance on Tuesday, which had been expected to be dominated by his battle with the trade unions.
Attacking politicians who earn "hundreds of thousands of pounds" as lawyers, he said: "The British people expect their MPs to be representing them and the country not anyone else."
Pundits were quick to point out one MP who would have fallen foul of any curbs - Ed's brother David Miliband, who declared £441,821.60 earned from speeches and Sunderland FC before recently quitting Parliament to work for an American charity.
According to an analysis by The Guardian in May, 20 MPs were making more from outside jobs than they were from Parliament.
It found Tory MPs had declared more than £4.3m in the Register of Members Interests, compared to Labour's £2.4m - which includes £1.36m for Gordon Brown alone.
Conservative MP Stephen Phillips is one of the top earners, charging clients about £600 per hour for his services as a commercial lawyer, The Telegraph reported - making him almost £1m since July 2011.
In May, he told The Guardian he was one of the "hardest workers" in Parliament.
"What matters is whether or not I do my job as an MP and how well I do it," he said.
Another lawyer, barrister and Tory MP Geoffrey Cox, declared £417,000.
High-earning Labour figures included former ministers Alistair Darling (£263,000) and Jack Straw (£183,000).
"We're not talking about leaving an MP in poverty," said James Graham, of the Unlock Democracy campaign, pointing to a member's basic £66,396 salary.
Graham said there was "some justification", for example, for a doctor to keep their hand in, but said it was "hard to fathom" why MPs should be allowed to hold highly-paid directorships.
"At the very least, where an MP is earning huge amounts of money, I can't see why we should be paying their MP's salary as well," he added.
A cap is a proposal that should chime with the public - polls have found a majority of people support an outright ban on outside earnings.
Miliband said the issue had to be tackled "if we are to rebuild trust in politics."
Some outside jobs were "paying higher salaries than the job of an MP itself," he said.
"Decades ago being an MP was often seen to be a second job.
"The hours of Parliament starting in the afternoon, so people could do other jobs in the morning.
"We have changed that.
"But there remains a problem, as recent episodes involving lobbying and outside interests have shown."
He went on: "The question of MPs second outside jobs has been discussed but not properly addressed for a generation.
"The British people expect their MPs to be representing them and the country not anyone else.
They understand that Members of Parliament need to keep connected to the world beyond Westminster and will always write articles and give speeches.
"But can it be right that the rules allow MPs to earn hundreds of thousands of pounds from private legal practice while they are supposed to be an MP?
"Or from outside corporations without any real form of regulation?"
- 'Give MPs a pay rise or there'll be another expenses scandal'
Miliband vowed to "change things" if Labour win the next election, but did not go into detail about the level of a cap on outside earnings.
Answering questions, he said while MPs' jobs were declared, "there's no checking, no regulation of the system."