The Electoral Commission says it will look into Lib Dem cabinet minister Chris Huhne's election expenses following a complaint.
The Sunlight Centre for Open Politics is disputing spending he declared on leaflets and an election website.
The commission says there "may be the possibility of an offence", but Mr Huhne said he was confident his expenses were in order.
On Friday, the commission rejected another complaint about his expenses.
Those allegations, made by two former Lib Dem councillors in Mr Huhne's Eastleigh constituency, were based on a recording of a local party meeting where treasurer Anne Winstanley is heard saying that the elections "have cost more than we declared".
But the Electoral Commission replied saying no "specific evidence" had been provided and it would not be taking any further action on the matter.
Separately, police are investigating claims that the energy secretary tried to evade a speeding punishment - an allegation he denies.
His estranged wife suggested someone close to him took licence points for him after a speeding offence committed on the M11 motorway in 2003.
Both Mr Huhne and his wife, Vicky Pryce, were interviewed by Essex Police last week over the allegations.
In a statement on Tuesday, the commission said it had received a complaint from Sunlight on 25 May.
"Following an initial assessment of the information, we have now started a case review into the matter.
"The review will look to establish the facts of the case, firstly for the purposes of transparency and also for possible future guidance."
In a letter to Sunlight - a group which campaigns for greater transparency in politics - the commission wrote: "Our assessment of the information indicated that there may be the possibility of an offence under Representation of the People Act 1983."
In response to the allegations, Mr Huhne said in a statement: "I have full confidence that my agent has declared my election expenses correctly and I look forward to this complaint being rejected as roundly as the last one."
The Sunlight Centre is expected to refer its complaint to the police.
Mr Huhne's electoral expenses declaration was made on 10 June 2010. For any legal proceedings to be brought under the Representation of the People Act they would have to be filed within 12 months of that date.
The commission said that given this time limit - and its estimate of up to 90 days for the completion of its review - its involvement would be to establish the facts only and not to take the matter any further.
It advised Sunlight that if it wished "to pursue this matter in terms of possible proceedings" it would need to make a complaint to the police within the time limit.