David Wilshire compares treatment of politicians to Jews in Nazi Germany
Mr Wilshire, who was forced to resign after paying more than £100,000 in expenses to his own company, said the "witch hunt" against MPs "will undermine democracy".
"Branding a whole group of people as undesirables led to Hitler's gas chambers," he added.
The Tory MP used his office expenses to write to all his constituents defending his claims and attacking The Daily Telegraph.
Voters in his Spelthorne constituency were surprised to receive a two-page letter, written on Commons notepaper and sent using taxpayer-funded pre-paid envelopes, in which he said that he was "devastated" at having to stand down.
A voter who emailed to protest at Mr Wilshire's behaviour received the reply in which Mr Wilshire compared MPs whose claims were exposed during the expenses scandal to Holocaust victims.
The response goes even further than Alan Duncan, the former shadow leader of the House, who was secretly recorded complaining that MPs were now forced to "live off rations" in the wake of the expenses scandal. Mr Duncan was later demoted from the Conservative front bench.
Mr Wilshire wrote: "The witch hunt against MPs in general will undermine democracy. It will weaken parliament - handing yet more power to governments. Branding a whole group of people as undesirables led to Hitler's gas chambers."
The email was sent four days after Mr Wilshire wrote to his constituents with his version of the events which led him to announce that he would stand down from Parliament at the election.
Earlier this month, The Daily Telegraph disclosed that he had paid £105,000 from his office expenses to Moorlands Research Services, a firm owned himself and his girlfriend.
He told his constituents that he had been "attacked" by the Telegraph, but was writing: "with a clear conscience".
Mr Wilshire confirmed that the letters were sent using House of Commons envelopes, which are available to MPs to enable them to "facilitate their parliamentary duties."
Adding that he was "entitled to explain" himself to his constituents, he said: "It is in accord with the rules."
But one recipient, Christopher Frazer, wrote back to the MP calling on him to quit Parliament immediately rather than staying on until the election.
He said: "Thank you for your letter dated October 26 about your MPs expenses.
"I enjoyed reading it, as will others: a more appropriate form of apology, however, would be to resign forthwith rather than mouthing on about having a 'clear conscience,' 'a very heavy heart,' feeling that 'I have let you down' and so on."
Mr Frazer, a Tory voter who stood as a Conservative candidate in the 1992 general election, said that he was glad that the MP was quitting, as he would not have felt able to support him at the general election.
House of Commons guidance on the use of free stationery says that MPs may send a maximum of £7,000 worth of pre-paid envelopes.
It goes on: "It is your responsibility to ensure that all expenditure funded under the Communications Allowance and the provision of House stationery and pre-paid envelopes is wholly, exclusively and necessarily incurred on your Parliamentary duties.
"You are responsible for ensure that your use of ... House stationery and pre-paid envelopes is above reproach."
Mr Wilshire defended his use of the envelopes, however, saying: "People want to know about what is happening.
"There was obviously concern, and because there was concern I wanted to contact them to let them know about the situation."
In his letter, the MP complained of being the victim of "false allegations" and insisted that his company was "a properly constituted business".
He added: "This use of allowances to help an MP do his/her job is completely within Parliament's rules."
John Lyon, the Westminster sleaze watchdog, has launched an investigation after Mr Wilshire referred his claims to him.
The MP said: "With a very heavy heart I concluded that standing down as your MP was the best way to help my constituents, our national party, you as its local members and my family.
"The investigations being made at my request by the Commissioner for Standards will take time.
"This would probably have meant an election campaign without the closure needed to put these allegations behind us.
"It would have been unkind of me to subject you to the type of local election campaign that would have taken place.
"Needless to say, I am devastated by having to give up a job I love and knowing that my 22 years of service will probably be remembered for unfounded allegations. I feel I have let you down."
I just... I... no words. The mind boggles. I don't know if some of these MPs live on the same planet as the rest of us.