Ex-ministers who work as lobbyists should have to go on a register for up to 10 years to ensure "transparency", the head of the government's appointments watchdog has said.
Lord Pickles said the rule, which he also wanted to apply to former civil servants, would "usefully limit the potential for any conflict [of interest] to arise".
The comments follow revelations involving former Prime Minister David Cameron, who lobbied current ministers on behalf of the financial firm Greensill Capital.
He has denied breaking any codes of conduct or any government rules.
On Tuesday, the Treasury select committee will hear evidence from Greensill Capital's owner, Alexander Greensill.
Former ministers are not allowed to lobby government - that is, try to persuade it to change policy - for two years after leaving office.
They are free to do so afterwards, and have to register with the Office of the Registrar of Consultant Lobbyists if they work for a firm that carries out lobbying on behalf of others.
But if they work "in-house" for a company they do not.
As an adviser employed directly by Greensill, Mr Cameron argues this meant he was exempt from having to register.
There are currently several inquiries going on into the extent of Greensill's lobbying activities and the workings of the wider system.
Lord Pickles, chairman of the Advisory Committee on Business Appointments, offers his suggestions in a submission sent to all of them.
He writes that the government "might usefully consider whether former ministers and civil servants should be on the register where they are employed in the capacity of an in-house lobbyist for a set period of time".
Lord Pickles adds that this should last for "two election cycles (or 10 years, whichever is greater)", as this would "usefully limit the potential for any conflict [of interest] to arise".
He also writes that "it is damaging to the integrity of government if former officials or ministers leaving office are seen to trade on their time in government, to be paid for access to and/or to influence the government".
Mr Cameron, who was prime minister from 2010 to 2016, started work for Greensill in 2018.
In 2020, he lobbied ministers unsuccessfully to allow the company to issue government-backed loans, as part of a scheme to help big firms through the pandemic.
This included texting Chancellor Rishi Sunak and contacting Treasury ministers Jesse Norman and John Glen.
Following widespread criticism when the details were revealed earlier this year, Mr Cameron admitted he should have operated "through only the most formal of channels".
But he said: "In my representations to government, I was breaking no codes of conduct and no government rules.
"The Registrar of Consultant Lobbyists has found that my activities did not fall within the criteria that require registration."