Search: Election vote queues
The chair of the Electoral Commission has described the system as "Victorian" after voters were left queuing in their hundreds outside polling stations when the ballot boxes closed.
The disruption came amid a high turnout with some polling stations running out of ballot papers as voters headed to the polls. With some results still to be declared, national turnout stood at 65%, up from 61.4% in 2005 and 59.4% in 2001.
Electoral Commission chief Jenny Watson said the UK voting system was at breaking point and promised a thorough review into problems around the country, including London, Sheffield, Manchester, Liverpool and Newcastle.
The leaders of the three main political parties have all condemned the situation.
Prime Minister Gordon Brown expressed concern at the reports of queues up to two hours long with people being turned away from voting at 10pm in the face of high turnouts and low staff numbers at polling stations. His spokesman said the Prime Minister was "very concerned by the reports and would support a thorough investigation into them".
Mr Cameron said a new government must "get to the bottom of what has happened and make sure that it never happens again".
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Liberal Democrat leader Nick Clegg said: "It is not right that hundreds later found themselves unable to exercise their vote when the polls closed. That should never, ever happen again in our democracy."
Ms Watson said that, after carrying out a review, the Electoral Commission would make recommendations to change the law.
Electors in the Liverpool Wavertree constituency were told they had to wait for new forms to be delivered before they could cast their votes. There were angry scenes at polling stations in Hackney, east London, where would-be voters staged a sit-in, and in Nick Clegg's constituency of Sheffield Hallam, students tried to prevent ballot boxes being taken after they were turned away.
In some places, including two polling stations in Newcastle, voters were ushered into the building before 10pm when the doors had to be shut, while at one site in Lewisham ballot papers were handed out to the queue before the deadline. But there were reports in the Manchester Withington constituency of people queuing for more than two hours before being turned away because the polls had closed.
