MPs are to be banned from employing spouses and other relatives using taxpayers' money.
Westminster sleaze watchdog the Independent Parliamentary Standards Authority announced the ban as it published a revised version of the rulebook for MPs' expenses claims.
Chair Ruth Evans said Ipsa will stop paying for new staff members related to MPs from the time of the next general election, scheduled for 2020. Existing employees with close family or business links to MPs - known as "connected parties" - will not be affected.
After 2020, a member of staff who begins a relationship with an MP while working for him or her will have their contract ended after two years as partner or spouse.
Ms Evans said: "We have decided to end funding for new employees who are related to MPs from the next general election onwards.
"We believe that the employment of 'connected parties' is out of step with modern employment practice, which requires fair and open recruitment to encourage diversity in the workplace."
Some 151 of the the 650 MPs at Westminster - almost a quarter of the total - employ family members, including 84 Conservatives, 50 Labour and 10 SNP MPs.
There was anger among MPs at the introduction in 2010 of a limit of one family member of staff in the wake of the expenses scandal, with many arguing that spouses were best able to handle the unpredictable work patterns, long hours and need for absolute trust associated with being an MP's secretary or assistant.
The change led to a sharp fall in the numbers of MPs employing family members, from 32.8% of male MPs and 23.1% of women in 2009 to 26.6% of male MPs and 12.9% of women in 2012/13.
In a review of the practice, Ipsa heard calls from members of the public and some MPs that it should be halted, while the Committee on Standards in Public Life argued it was "not appropriate in a modern workforce".
But the majority of MPs supported the continuation of existing rules, with one group of Conservatives arguing that spouses represented "good value for money as they often work well beyond the hours that other staff would be prepared to work".
Despite finding there had been "no significant compliance concerns" since the introduction of the new rules, Ipsa decided that employing relatives was "out of step with modern employment practice, which requires fair and open recruitment and management of staff". Ending the practice would "help to encourage greater diversity among MPs' staff", the watchdog said.
Citing guidelines from the Advisory, Conciliation and Arbitration Service (ACAS) and the Equality and Human Rights Commission (EHRC), Ipsa said: "We acknowledge the need for MPs to employ people they can trust, but do not consider that these can only be connected parties."
"We are clear that this decision is not based on any identified abuse or mis-claiming. We agree with the MPs who have told us that connected parties regularly go 'above and beyond' in supporting MPs in their constituency.
"But we have nonetheless determined that, in order to encourage good practice in recruitment and employment by MPs, and to assure the public about the appropriate use of taxpayers' money, we should restrict the practice in future.
"On balance, the need for good employment practice which is transparent and encourages diversity outweighs the benefits which some MPs find in being able to employ connected parties."
Thanet North MP Sir Roger Gale, whose wife Suzy has worked as his office director since he entered Parliament in 1983, described Ipsa's decision as "crass".
"It will be incredibly damaging if we go down this route," Sir Roger told the Press Association. "Superficially, this is an easy hit for anybody wanting to show they are doing something. Actually, it is crass and they will live to regret it - and so will the House and its MPs."
The benefits of employing a spouse are "enormous", said Sir Roger. "Obviously, I trust her implicitly, she knows what I think and why I think it and constituents can call on her whenever I'm not around.
"She probably does 60 to 70 hours work a week, including most of Saturday, and I can call on her any time day or night. I couldn't do that with an assistant recruited through an advert."