With many thousands prevented from voting in marginal constituencies, this election is invalid and must be reheld within a month.
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.
Postal voting rules must be re-written, and those for proxy votes.
Cameron should head straight to court.