London Elects has promised to investigate allegations ballot papers were spoilt by election officials in Croydon yesterday.

A BBC journalist has claimed his ballot paper was spoilt by a polling official at the Oval Primary School polling booth in Cherry Orchard Road who wrote his voting card number on the back of the ballot paper.

A spokesman for the election organiser said it was looking in to the report but had not yet found any evidence of ballot tampering in Croydon.

He said a change of ballot paper format could have confused some staff and led them to mark yesterday's votes, despite being told in training not to.

In past years, voting cards had carried numbers that were torn off and collected separately from the ballot papers.

"The members of staff involved will never be employed at polling stations again," he said.

"We take this very seriously and apologise to anyone whose vote was affected."

Croydon returning officer Jon Rouse said there had been no voting irregularities reported in the borough.

"We are at the count and there is no evidence of irregular practice so far," Mr Rouse said in an email.

A Croydon Council spokesman added: "We can categorically say that no ballot papers have been tampered with from the polling station in question.

"The ballot box has now been opened to count and no polling cards have been tampered with by polling station staff."

London Elects officials ordered an investigation in to a similar incident in Lambeth during yesterday's mayoral and London Assembly vote.

London Elects said then it was investigating what it described as an "isolated incident" in Lambeth where polling clerks had written what is thought to be the voting card numbers on 10 people's ballot papers.

A spokesman said any mark other than the voter's cross on the ballot paper made it invalid.