Zac Goldsmith's election expenses are under investigation, after the Electoral Commission confirmed it was assessing a dossier that raised questions about the Richmond Park MP's campaign spending.

An investigation by Channel 4 News and the Bureau of Investigative Journalism alleged the full cost of posters, leaflets and jackets, as stated on invoices, was not declared as election spending in returns submitted to the returning officer.

Strict spending limits capped Richmond Park candidates' costs in the last 23 days of the election campaign to £11,003, and Mr Goldsmith declared spending of £10,783.

The dossier claimed the cost of some signs was attributed to local election candidates, and the cost of materials used before the election and leaflets that were not delivered were not declared.

Mr Goldsmith said: "We were scrupulous at every stage in ensuring that our election expenses complied absolutely with the letter and the spirit of the rules. If the Electoral Commission looks into it, it will find nothing out of order.

"We followed precisely the same formula used by MPs and candidates across the country. We were probably even more careful than most, because it was a high profile campaign in a hotly contested seat, and it was inevitable that we would be examined afterwards.”

Councillor Derek Osbourne, Liberal Democrat leader of Kingston Council, said: “It's confusing to me if the allegation is true that a poster with a photograph of one politician and with one name on it can be switched to the election expenses of candidates whose photographs do not appear and whose names are not mentioned.

"It is always a matter of concern when there are accusations that individual politicians might have broken electoral law to buy an election through unfair spending.

"All such accusations should be investigated.

"As with MPs' expenses, the accusation of a relatively small number of transgressors brings a bad repuation on all politicians both national and local."