The chairwoman of the UK's Electoral Commission described the current system as “Victorian” today 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 on mass.
EC chief Jenny Watson said the system was at breaking point and promised a thorough review into problems around the country, including London, Sheffield, Manchester, Liverpool and Newcastle.
After results in 603 of the UK’s 650 constituencies had been announced, national turnout stood at 65%, up from 61.4% in 2005 and 59.4% in 2001
The leaders of the three main political parties have all roundly condemned the situation which led to the chaotic scenes.
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”.
Liberal Democrat leader Nick Clegg said the situation was simply not acceptable in a democracy.
He 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.
She told GMTV: “We’ve been calling for better co-ordination, for power of direction for us to tell people what to do, for some time. We haven’t been successful in persuading Government and Parliament to take that up so far.”
She added: “I hope that last night’s experiences will persuade them there is a real need to look at our Victorian system and modernise it fit for a 21st century democracy.”
The high turnout meant that in some areas election staff ran out of ballot papers.
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 after they were told they could not vote, and in Nick Clegg’s constituency of Sheffield Hallam, students tried to prevent ballot boxes being taken to the count 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.
Ms Watson said that if there were constituencies which had not followed the rules, they could be subject to challenges.
She said returning officers would have to “answer to us and answer to local voters”.
She told BBC News: “The law is extremely clear. They have the guidance. They should have done what the law says.
“If they haven’t done that... they may well be subject to election petitions.”
Labour’s deputy leader, Harriet Harman, said: “If there is any close outcome that is going to be produced by that, there should be a legal challenge – and quite right too.
“It is fundamental that people get their right to vote.”
Arriving for the count at her Camberwell and Peckham constituency in south east London, she said: “It is outrageous that people who were there long before 10pm at some polling stations have not been allowed to vote.
“They should have been issued with their ballot papers and been allowed to cast their ballot after 10pm.”
Tory Party chairman Eric Pickles commented: “It’s ridiculous. Of course people should be able to vote.
“Surely to goodness the returning officers could have just put the people in the polling station and continued.”
Already, the two Labour parliamentary candidates in Hackney, where residents were voting for their MP, local councillors and borough mayor, have launched an official complaint over their supporters’ inability to vote.
And in Sheffield, the returning officer, John Mothersole, apologised to voters for getting it “wrong”.
He said: “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.
“This made the administration process of ensuring the correct person was given a ballot paper much longer.
“The only remedy, which we could not take, was to extend the voting times,” he said.