Tory MP Derek Conway has defended parliamentary payments to his son, for which he was censured and suspended, saying: "I am not a crook."
Mr Conway was reprimanded by a parliamentary committee
A Commons standards committee said there was no record of Freddie, a student, doing work at Westminster in return for £40,000 of taxpayers' money.
Alistair Graham, the former chairman of the committee, criticised the MP for Old Bexley and Sidcup's defence.
But he said it showed the system is "pretty rotten".
Sir Alistair told BBC News: ''I think his general argument is, I've just been a fall guy for a system in which many other MP's have done similar sorts of things to myself.
"I never think that's a very good defence of having done something wrong but it really does seem to demonstrate that the system is pretty rotten."
Mr Conway has been suspended from the Commons for 10 days and ordered to return £13,161 of the money he paid his son.
He announced he will stand down as an MP at the next General Election, after Tory leader David Cameron withdrew the Conservative whip following the report.
But in an interview with the Mail on Sunday, Mr Conway said: "I still believe I have done nothing wrong."
He said his son travelled from Newcastle to help him with his MP work.
He told the paper: "I know many MPs with family members who have different names registered, so they are not so obviously spotted. Some spouses work under maiden names."
Mr Conway insisted both Freddie, 22, and elder son, Henry 25, whom he employed earlier, did the work they were paid for.
"A lot of students do part-time work. He was working for his father rather than working in McDonald's," he said.
"He used to come home frequently. He would go up and down like a fiddler's elbow while he was away.
"There are MPs who commute greater distances than that on a weekly basis and some are three times Freddie's age. I don't think it was unusual."
Mr Conway may face a second inquiry into the job he gave Henry, while Scotland Yard are considering a demand for a police probe into allegations of fraud.
The standards committee report, which said Freddie was "all but invisible" at Westminster, concluded the arrangement was "at the least an improper use of parliamentary allowances" and "at worst, a serious diversion of public funds".
Mr Conway said Freddie and Henry worked from the family flat in nearby Victoria, adding: "Lots of MPs have family who work from home."
Outlining the work Henry did, he said: "He'd fillet post, scrutinise e-mails and stuff envelopes. I am one of many MPs who employ family members. It doesn't mean there's not a job to be done or that they weren't doing it."
Mr Conway also denounced as "homophobic" media reports which have referred to Henry's sexuality and rejected allegations he employed another researcher, Michael Pratte, because he was a friend of his sons.
Mr Pratte met Freddie and Henry only after joining his office, he said.
The MP also questioned the motives of some of his parliamentary critics, suggesting Labour was exploiting the issue to distract the public from the donations row.
"This was a golden opportunity - clearly supported by some Tories, some of whom may well have had their own agenda," he said.
The row has prompted the leaders of all three main parties to tell MPs to make it clear if they employ relatives.
Former cabinet minister Peter Hain has since confirmed that his mother works for him as a part-time secretary.
Adelaine Hain, 80, has worked for her son at the Commons since 1991 and was paid £5,400 last year.
Mr Hain, who quit as pensions secretary last month over a probe into undeclared donations, said: "I've never hidden anything. She works really hard."