Investigation as voters turned away at polls
Gordon Brown said he was "very concerned" by the matter and politicians from all sides called for a thorough investigation.
With polling stations unable to cope with demand, thousands of people were still queuing when ballot boxes closed at 10pm last night.
It may lead to by-elections in the next few weeks, which could be critical to the outcome of the election.
Voters were turned away in Hackney, Islington, and Lewisham in London.
People in Sheffield, Manchester, Liverpool, Chester, and Newcastle were also affected.
Sky News sources said 200 people were turned away in Manchester Withington alone.
There were reports of similar situations in other parts of the country.
Voters in Hackney said they were turned away from a polling station in Triangle Road after some had been forced to queue for more than an hour and a half.
They staged a sit-in protest at the building and police had to be called.
And officers were called to a polling station in Manwood Road, Lewisham, where around 300 people had yet to vote by 10pm.
Police were also sent to Sheffield Hallam, where Lib Dem leader Nick Clegg was standing, after students refused to let ballot boxes out of the building.
Sheffield’s deputy returning officer Lee Adams said around 200 people were turned away and admitted staff "couldn’t cope" with the numbers who had turned up to vote.
The Electoral Commission said it would be undertaking a "thorough review" into the chaos and called it a matter for "serious concern".
A spokesman said there should have been sufficient resources allocated to make sure everyone was able to vote.
Chair of the Electoral Commission Jenny Watson was questioned on Sky News about whether she should resign.
She told Sky’s political editor Adam Boulton that her commission was not to blame for the mess and that "decisions are made by returning officers who are entirely independent".
She said the law is that doors to polling stations must be closed at exactly 10pm, and that no one may be issued with a ballot paper after 10pm - even if they are inside the polling station.
But Simon Nayyar, Conservative candidate for Hackney South and Shoreditch, said some voters in his constituency were being turned away at 9.15pm because queues were so long.
He said he was "extremely disturbed" by the problems and would be calling for a "full public inquiry".
The National Union of Students (NUS) claimed students in some areas had been unable to vote after they were placed in separate, slower queues to other voters.
NUS president Wes Streeting said: "Where students and other voters have been disenfranchised, local authorities should hang their heads in shame.
"It is outrageous that citizens should be denied their basic right to vote and we demand an inquiry into how this situation occurred."
A spokesman for Gordon Brown said: "The Prime Minister is very concerned by the reports and would support a thorough investigation into them."
A Conservative party spokesman said: "These are very disturbing stories which clearly need to be thoroughly investigated."
Asked if the situation amounted to a "scandal", Olympics Minister Tessa Jowell replied: "I think it is.
"These are queues of people exercising their democratic right and then being denied it."
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