The new MPs' expenses system risks creating a "tragedy" by excluding poorer people from political life, a watchdog has warned.
Sir Christopher Kelly, chairman of the Committee on Standards in Public Life, said "insufficient attention" had been paid to supporting parliamentarians.
He said in particular people with young families could struggle to become MPs in future unless changes were made.
A list of 125 MPs whose claims had been rejected was published last week.
The body responsible - the Independent Parliamentary Standards Authority (Ipsa) - took control of the administration of MPs' allowances last year, following the expenses scandal of 2009.
Sir Christopher said its creation had been "one of the most important steps taken to restore public confidence".
But there was a risk that "insufficient attention" had been given to the primary function of the expenses regime: supporting MPs in doing their "important and difficult jobs".
Sir Christopher said: "The committee has seen much anecdotal evidence - including from their own conversations with individual MPs - that the current scheme as presently constituted is not yet succeeding in fully meeting that objective, even allowing for inevitable teething difficulties.
"It would be a tragedy if the implementation of an expenses scheme were to have the effect of inadvertently and unnecessarily limiting access to the role of MP for those with young families, caring responsibilities or other challenging personal circumstances."
Sir Christopher blamed the way in which the new body had been set up for the potential problems and said its rules may need to change to reflect the reality of MP's daily lives.
He told BBC Radio 4's Today programme: "Ipsa was set up in a hurry, it was set up on the back of a scandal, a lot of ill feeling in the public mind about what MPs did.
"It's very important that one should never forget that the prime purpose of an expenses scheme for MPs, as it is for any other group of people with an expenses scheme, is to help them do their jobs properly.
"And there's enough anecdotal evidence and evidence from those MPs who talk to us to suggest that there are a number of things where IPSA may want to think again."
Since its introduction, Ipsa has been criticised by MPs for what they say are its excessive running costs and bureaucratic restrictions on claims.
In December, David Cameron told a meeting of Tory MPs the new expenses rules were "anti-family" and causing a "lot of pain and difficulty".
His aides said he was referring to issues such as MPs not being able to use expenses to transport their children to and from their constituencies.
The prime minister said he wanted to see a better system in place by April.
Sir Christopher said he hoped Ipsa would be prepared to modify the system where there was evidence that it was failing.
And it should also be prepared to give MPs advice on the implications of the new regulations - even though it was ultimately down to politicians to ensure claims were legitimate, he added.
Ipsa has said it may reconsider some aspects of the system, but adds that it is independent and will not be rushed into changes.
On Tuesday, Commons Speaker John Bercow also said the rules were "threatening both MPs' family lives and their capacity to discharge their duties as they fully wish".
He added: "Relatively modest changes to policies on accommodation, travel, the rental of offices and the staffing of them, combined with a reasonable flexibility in the use of members' budgets, would yield real benefits to members and constituents alike."