Ms Kirkbride and Ms Moran had been under intense scrutiny
Tory MP Julie Kirkbride and Labour's Margaret Moran are standing down at the election after expenses revelations.
Ms Kirkbride had faced a welter of claims about her expenses claims and said she "must take into account the effects on my family" of the row.
Ms Moran, who has been under pressure over her £22,000 dry rot claim, said she was quitting "with great sadness".
Tory MP Christopher Fraser said he will also step down - but insisted it was not connected to his expenses claims.
And senior Tory MP Bill Cash has been forced to defend his expenses claims after the Daily Telegraph reported he paid his daughter £15,000 in rent from his parliamentary allowances - something he insists was approved and within the rules.
In a letter to Mr Cameron, Ms Kirkbride, 48, said she wanted the Conservatives to have a "great result" at the next election.
She added: "My principal concern has to be for my very loyal local supporters in Bromsgrove whose trust in me has been very humbling in the last few weeks.
The understandable anger in the media and amongst the public over the issue of my Parliamentary expenses has had a bruising effect upon my friends, my family and my health
"I also must take into account the effects on my family."
In his reply, Mr Cameron thanked her for being so "frank and candid", adding that he appreciated that she had been "under enormous pressure" from "unbearably intense" media scrutiny.
He said: "I know this was a very hard decision for you to take, but I completely understand why you have decided to do so.
Ms Kirkbride had been due to take the fight for her political future to the doorsteps of her Bromsgrove constituency over the next few days but BBC political correspondent James Landale said there was a clear sense that the row had to stop.
Ms Kirkbride was initially embroiled in the expenses story as a result of revelations that her husband - and fellow Tory MP Andrew Mackay - had named their main home in London as his second home and claimed expenses for it.
But despite him quitting last week, she had to answer questions about her brother, who lived rent-free in her taxpayer-funded constituency home and her sister, who did secretarial work for her despite living more than 100 miles from her constituency.
David Cameron: 'I respect Julie Kirkbride's decision to step down'
She also came under fire for re-mortgaging her second home to fund an extension and claiming for the higher interest payments.
She said she needed an extra bedroom so her eight-year old son did not have to share a room with her 59-year old brother, who helps to look after him.
Before announcing she would quit, she told BBC Radio 5 Live that she understood public anger over the expenses row.
But she added: "Until this week's furore, it didn't cross my mind that I had done anything wrong."
Ms Moran's decision to stand down came amid speculation about her future after it emerged she claimed £22,500 for treating dry rot in her designated second home in Southampton - 100 miles from her Luton South constituency.
Although she had repaid the money a Labour panel had been due to decide whether she could stand for the party again.
Ms Moran, 54, said she had taken the decision in the interests of her family but insisted she had done nothing "wrong or dishonest" in her expenses claims.
MPS LEAVING PARLIAMENT
The following MPs have said in the past three weeks that they will not contest the next election
Conservative: Andrew MacKay, Julie Kirkbride, Douglas Hogg, Sir Peter Viggers, Anthony Steen, Sir Nicholas and Ann Winterton, Christopher Fraser
Labour: Margaret Moran, Ben Chapman, Ian McCartney, John Smith, Michael Martin (Speaker)
A few hours later Christopher Fraser, MP for South West Norfolk, said he had decided not to fight the next election, saying that his wife's on-going health problems had "made it difficult to juggle my family life with my duties as an MP".
The Daily Telegraph said Mr Fraser had claimed £1,800 in public money to buy more than 200 trees in his garden and he has faced questions in his constituency over his designation of a home in Dorset as his main home.
And in another development, Employment Minister Tony McNulty repaid more than £3,000 in mortgage interest and council tax "mistakenly" claimed for a house in Harrow - which he nominated as his second home for expenses - where his parents lived.
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