Thousands of voters were turned away before they got to the ballot box as understaffed and ill-prepared polling stations struggled to cope with the high turnout.
More than 300 people were blocked from a polling station in Lewisham. A further 150 were turned away in Hackney South, with some being told as early as 9pm they would not be able to cast their vote. Other affected areas include Ealing, Islington, Surrey, Sheffield, Leeds, Manchester, Birmingham, Liverpool and Newcastle. Lib Dem leader Nick Clegg personally apologised to people in his Sheffield constituency who had queued for more than three hours.
Tempers flared as people demanded the right to cast their votes and police were called to ensure the scenes did not descend into violence.
It also emerged that several polling stations ran out of ballot papers, forcing officials to turn people away.
In another disaster, key marginal seat Chester failed to update its electoral list, meaning 600 people who had registered were not allowed to vote. The seat, held by Labour MP Christine Russell, was decided by just 973 votes in 2005.
The voting chaos raises the prospect of legal action from losing candidates who may claim their results were unfair.
Jenny Watson, chair of the Electoral Commission, said she had “very serious concerns” over the way the election had been handled.
“We will look at this and parliament will look at this. The system we have is at breaking point. It is a legacy of the Victorian era when very small numbers of people voted.
She called for greater powers for the Electoral Commission and added: “The law will have to be reviewed and changed.”