A Tory MP has said he is "mortified" to learn part of a £5,822.27 expenses claim covered the electricity supply to his stables.
Nadhim Zahawi, a founder of market research firm YouGov, has pledged to repay the money which came to light after he was among politicians criticised over subsidised energy bills.
The Stratford on Avon MP claimed the most of those highlighted to cover electricity and heating oil for his estate in Warwickshire.
In a statement posted on his website, he said: "Since last week's coverage of my energy bills I have been looking into them further and can confirm that all claims for heating fuel relate purely to my second home.
"However I have made a mistake with my electricity claims.
On investigation I have discovered that the electricity supply for a mobile home located in the stable yard and for the stables themselves was linked to my house.
"Whilst a meter was installed in the stable yard I have only been receiving one bill, it was wrong to assume I was receiving two and to have not checked this sooner.
"I am mortified by this mistake and apologise unreservedly for it.
"I will obviously be paying back any money that was wrongly claimed immediately and have already contacted IPSA (Independent Parliamentary Standards Authority) regarding the correct procedure."
Some 340 MPs, including government ministers, have used the parliamentary expenses system to recoup the cost of heating their second homes, it was reported.