Former Tory MP Nadine Dorries says she will return £16,876 in severance pay she mistakenly received for being a cabinet minister.
Government accounts show she received the exit payout from her tenure as culture secretary under Boris Johnson.
But the rules say she should not have received it, because she had reached the age of 65 when she left office.
Ms Dorries said she only received an email about the admin error on Friday.
Speaking to BBC One's Sunday with Laura Kuenssberg, the ex-minister said she would repay the money "on Monday morning".
She added that she was "gutted" to find out, joking that the mix-up "means now everybody knows I'm not 49".
Former ministers are entitled to receive a severance payment equivalent to three months of their annual ministerial salary when they leave office.
But only those who are younger than 65 when they leave their post, and are not reappointed as a minister for three weeks, are eligible for the money.
Ms Dorries left her role as culture secretary under former prime minister Mr Johnson in September 2022, around three months after she turned 65.
Her erroneous exit payment was listed in her old department's latest set of annual accounts, but was first reported by the Times earlier this week.
It is one of five exit payouts the government appears to have mistakenly made to former ministers aged 65 and over in the course of the last financial year.
Departmental accounts show Tory peer Baroness Stedman-Scott was paid £17,742 after leaving as a pensions minister in December 2022.
Former Tory MP Peter Bone, who was ousted by local voters as the MP for Wellingborough in December, was paid £5,593 for his three months as deputy Commons leader in 2022.
Former health minister Maggie Throup received the same amount, while former whip Sir David Evennett was paid £4,497.
On Thursday, the Cabinet Office said relevant departments would be contacting those who erroneously received payments to recover them.
Sir David told the Times he had repaid the money he had mistakenly received after being alerted to the error this week.
It is understood that Baroness Stedman-Scott has contacted her old department about repaying her mistaken severance pay, and was unaware of the error until it was reported in the press.
The BBC has contacted Ms Throup and Mr Bone for a comment.
Ms Dorries said there were "no details" in the email she received about how she could repay the money, but she was sure she would "find out". It is understood her former department's HR division is liaising with her.
Labour has pledged to revamp ministerial severance pay rules if it wins power, by linking the amount received to the actual time spent in office, thereby reducing payouts to ministers who spend less than a year in their roles.
Ms Dorries entered Parliament as the MP for Mid Bedfordshire in 2005, and was a minister at the health department before being promoted to culture secretary by Mr Johnson in September 2021.
One of the former premier's staunchest political allies, she formally quit as an MP in August last year with a blistering attack on current PM Rishi Sunak.
It came two months after she announced she would quit "with immediate effect," with the former minister saying she wanted to stay in the role whilst she tried to find out why she was denied a peerage.