Blundering election officers at Hammersmith and Fulham council have sent letters to residents urging them to vote by May 19 – nearly a fortnight after the final count.

Claire Harris, a housewife from Barclay Road, Fulham, was left stunned by the correspondence, which came in response to her enquiry about how she could vote by proxy on behalf of daughter Iona, 22.

Mrs Harris, 54, said: "At first I thought I was going bananas – I just didn't think such an error could be made. How on earth could such a mistake be made at such an important time?"

The council says the bungled instruction – which tells people to get their vote in by Tuesday, May 19, a date which doesn't even exist – was only sent to eight households.

But Iona believes such a mistake is still unacceptable, saying: "I would like to think most people would have realised this was a mistake but some possibly wouldn't and it could have cost them their vote.

"It just seems extraordinary that something as vital as this which is going through such a big authority wasn't checked and picked up by someone. It makes you wonder what else they get wrong."

A council spokesman said: "Hammersmith & Fulham Council is one of just three 'beacon councils' for electoral services nationwide. This letter was sent by error to eight residents who asked for a proxy application form and we apologise for the confusion caused. A voter guide is being delivered to all households in the borough this weekend and this shows the correct dates."