The voting system was condemned today as a throwback to the Victorian era as polling stations closed while hundreds of people were still queueing up to vote.
Jenny Watson, chairman of the Electoral Commission, promised a comprehensive review of the system after describing it as close to breaking point.
She dismissed it as "Victorian" amid growing anger at a lack of resources at several polling stations around Britain which left many voters unable to cast their ballot.
In chaotic scenes around Britain, some polling stations remained open for an extra half an hour, while others allowed no leeway at all and shut their doors. Elsewhere, voters complained of ballot papers running out and names being left off the electoral roll.
And amid growing anger at the number of people who were prevented from voting because of failings in the system there were several calls for a legal challenge.
Ms Watson said the Electoral Commission had warned for some time that the "system is at breaking point" and "clearly the law will need to be reviewed and changed".
A spokesman for Gordon Brown said: "The Prime Minister is very concerned by the reports and would support a thorough investigation into them."
Police were called in Sheffield, Manchester, Hull, Leeds, Birmingham, Newcastle Upon Tyne and several London locations as late-arriving voters found themselves locked out.
In the marginal seat of Chester, where Labour has a majority of just 973, Labour party officials claimed that more than 600 people were turned away because polling lists had not been updated.
At Nick Clegg's Sheffield Hallam constituency, voters who had been turned away went to protest outside the Liberal Democrat leader's house. At the polling station in Ranmoor, where Mr Clegg cast his vote, police were called as angry voters refused to let ballot boxes leave the building.
Mr Clegg later personally apologised to voters in his constituency who were denied the chance to vote and said: "I share the bitter dismay of many of my constituents who were not able to exercise their democratic right to vote in this election.
"I visited the busiest polling station this afternoon, and in mid-afternoon there were hundreds - particularly of young voters - who were queueing for an hour and a half to vote, and I think it is just not right that hundreds later found themselves unable to exercise their right when the polls closed.
"That should never, ever happen again in our democracy."
According to Electoral Commission rules, voters served with a ballot paper by 10pm must be allowed to vote. Ballot papers are only given once voters have reached the front of any queue and their names crossed off.
Helen Jackson, the former MP for nearby Hillsborough, said the situation potentially left the result open to legal challenge. John Mothersole, returning officer for Sheffield, said: "We got this wrong and I would like to apologise. We were faced with a difficult situation with the numbers of people, and a large amount of students turning up to vote without polling cards.
In Islington South, where in 2005 Labour won with just 484 votes more than the second-placed Liberal Democrats, police were called after a number of Green Party activists demanded to be allowed to register their vote after being turned away.
In neighbouring Islington North, Green candidate Emma Dixon said she would consider legal action after voters were removed from the Highbury West polling station, where the Greens' only councillor, Katie Dawson, was up for re-election. Voters in Hackney, east London, were turned away after some had queued for more than an hour and a half. Police were called to remove protesters, some of whom it was claimed were told they would not be able to vote as early as 9.15pm.
Police were also called to a polling station in Manwood Road, Lewisham, south London, where around 300 people had yet to vote by 10pm, Scotland Yard said. A spokesman for Lewisham Council said that two polling stations remained open later than 10pm, but voters were issued with ballot papers before that time, in line with the law.
In Manchester Withington, 200 people were said to have been locked out when the polls closed.
In Sutton Coldfield, in the West Midlands, there were reports that voters had been locked inside the Mere Green Polling Station at 10pm as the queues were so long. In Merseyside, people said polling stations had run out of ballot papers. Birmingham City Council were forced to close two polling stations and turn voters away.