Fewer than 10 MPs are likely to be affected by Boris Johnson’s proposed rule changes on second jobs, analysis of the register of interests suggests.

It came as the prime minister conceded it was a “total mistake” to back disgraced MP Owen Paterson, admitting to backbench MPs on the 1922 committee “on a clear road I crashed the car into a ditch”.

On Wednesday MPs voted 297 to nil to back Downing Street plans to restrict outside work to “reasonable limits” and prohibit parliamentary advice or consultancy, with Labour abstaining. Final details are to be drawn up by the cross-party committee on standards.

But analysis of what is known so far of the rule changes suggest their impact on the 99 MPs who hold second jobs could be severely limited, with only a handful of MPs affected.

On Wednesday the cabinet minister Anne-Marie Trevelyan, in a series of interviews, suggested the changes could mean a restriction on paid outside work limiting it to fewer than 20 hours a week, or below 10-15 hours a week, or just to eight hours a week.

A 20-hour weekly limit on outside work would only cover Geoffrey Cox, the former attorney general, who has been under fire over lucrative legal work spanning more than 1,000 hours a year which involved his voting by proxy from the Caribbean. Such a limit would theoretically let Cox cut back his hours and retain his main outside work for the British Virgin Islands, for which he is paid £400,000 a year for 40 hours a month.

Some MPs who work as councillors or mayors could also be affected, as could ministers, unless there were an exemption for jobs that counted as political service.

A 15-hour weekly limit would also conceivably cover Dan Poulter, a Tory MP who works as a doctor, and Andrew Murrison, a Tory MP working as a naval reserve surgeon, who helped with the coronavirus vaccination effort.

A 10-hour limit would drag in a few more MPs, including John Redwood, a former cabinet minister, who has been working about 12.5 hours a week as chair of the investment committee of Charles Stanley, earning £48,222 a quarter.

Many MPs earn high wages for a small number of hours, so would be out of reach of the proposed changes. Julian Smith, the Tory former chief whip, earns £2,000 a month for only one or two hours’ work advising on business development for Simply Blue Management.

The proposed prohibition on MPs being parliamentary advisers appeared to be so narrowly worded that only two Tories in 48 MPs with consultancy jobs directly fitted that description, according to the register of interests.

Philip Davies, the Tory MP for Shipley, is listed as being a parliamentary adviser on pawnbroking to the National Pawnbroking Association, getting £1,000 a month for five to 10 hours’ work.

Laurence Robertson, the Tory MP for Tewkesbury, is also a parliamentary adviser on sport and safer gambling to the Betting and Gaming Council, receiving £2,000 a month for 10 hours a month.

Only a few more MPs mention politics in the description of the advice that they offer as consultants. These include Stephen Hammond, a Tory former transport minister, who has been a strategic adviser to Darwin Alternative Investments, earning £60,000 a year for providing political advice on business and finance.

There is also a possibility that the work of James Gray, a Tory MP, could be covered, since he records having received £1,100 from Electric Airwaves for having helped train witnesses going before a parliamentary select committee hearing.

Most MPs working as consultants describe their work as advisers in general terms, offering “strategic advice” or business consultancy to private companies.

Downing Street sources insisted it was impossible to say how many MPs would be affected by the new rules, with the committee on standards in charge of drafting the changes.

But it appears Owen Paterson, the former MP for North Shropshire, who resigned for breaching paid lobbying rules, might not have been covered by the ban. He was described in the register as a consultant to Randox Laboratories, a clinical diagnostics company, getting £100,000 a year, and as a consultant to Lynn’s Country Foods, a processor and distributor of sausages, earning £12,000 a year. Paterson was doing less than five hours a week in the jobs, so would have been unlikely to have been hit by time limits.

There was much confusion among Tory MPs about whether their other jobs would be disallowed under the proposals put forward by Johnson. One former minister said he had put a few other roles “on ice” while he waited for clarity. He also criticised as “ridiculous” comments made by Trevelyan on Wednesday morning when she repeatedly offered a different figure for the number of hours each week it was acceptable for an MP to spend on another job.

Labour said it would “properly ban” second jobs rather than accept what it called the “watered-down cop-out” of the government’s approach.