American citizens living in the UK have attacked the US postal voting system after struggling to resister for the US election.

Around 250,000 US citizens are estimated to live in the UK and their votes could prove crucial with presidential candidates Barack Obama and Mitt Romney neck-and-neck in the polls.

Yet complex state-specific rules and "ridiculous" communication from election officials has led to accusations that the system is fundamentally disenfranchising American voters living abroad, including military personnel.

Democrats Abroad, an organisation that campaigns for the president but registers voters of all political persuasions, said they have received hundreds of calls from Americans in Britain struggling to vote.

Omar Siddiqui, a 25-year-old investment banker from Birmingham, was born in the key swing state of Ohio but has been refused registration, despite believing he voted in 2004.

Mr Siddiqui had his request for an absentee ballot rejected by officials at Butler County, near Cincinnati, after his US passport and UK driving licence failed the criteria for "photo identification" because they did not show a current American address.

A spokesman for Democrats Abroad said Mr Siddiqui provided every documentation required by law. The case was taken up by law firm Edwards Wildman, which wrote to Butler County on Friday insisting Mr Siddiqui be granted an absentee ballot within 24 hours. No response has yet been received.

"I don't understand why it has been stalling for so long," Mr Siddiqui said. "Election day is today, I was well ahead of the deadlines that I had to meet. I have given them everything, I have been very proactive in emailing. I haven't let it go and I have just been met with dead ends.

"I'm really astounded that, especially in such an important state, there's so much negligence. It is just incredible. This is America in the 21st century and people are being denied votes. It is just unacceptable. I'm really furious about this."

He continued: "This kind of stuff goes on in democracies that have just started or corrupt countries. You could never, ever expect this to happen in the United States."