Police have confirmed they are investigating a complaint of an alleged breach of election expenses rules by Scotland's only Tory MP, David Mundell.

It follows a weekend report in the Sunday Herald that a bill for £700 had been omitted for Mundell's campaign costs.

Dumfries and Galloway constabulary said it had been asked to investigate.

Mundell, a junior Scottish minister, said a mistake had been made in completing expense forms but this was due "solely to human error" and was "not any attempt to mislead".

"Such mistakes are not uncommon in election returns and there are established procedures for dealing with them," he said.

"Obviously, as soon as it was drawn to my attention, I informed the electoral commission who have an interest in such matters."

A statement from Dumfries and Galloway constabulary confirmed it was investigating the situation.

The force said it had received a formal complaint in relation to an alleged breach of the Representation of the People Act in the run-up to the election by Mundell.

The Sunday Herald reported at the weekend that the MP for Dumfriesshire, Clydesdale and Tweeddale was facing a possible inquiry over payment for a newspaper advert.

Like every candidate in the election, Mundell's spending was split into two parts. The first part, known officially as the "long campaign" period, ran from January 1 until the dissolution of parliament on 12 April.

Mundell spent £29,148 out of the maximum £29,664 allowed for his constituency, the Herald reported.

In the second "short" campaign period, between April 13 and polling day on 6 May, he spent another £11,599 out of a maximum £11,814.

The cost, nearly £700, would have taken his expenses over the permissible limit.

The newspaper said it appeared the money had been counted, but was attributed to an earlier stage in the campaign. According to electoral commission rules, no candidate is allowed to spend more than the cap in either the long or short period, and spending in one period cannot be swapped to the other to stay within the limits.

The cost, nearly £700, would have taken his expenses over the permissible limit.

The newspaper said it appeared the money had been counted, but was attributed to an earlier stage in the campaign.

The electoral commission confirmed Mundell was in touch at the beginning of the week to "alert us there may be a problem", though it added that the weekend reports had already triggered an initial assessment of the matter, which will conclude on Monday.