The elections watchdog has demanded "urgent changes" after condemning failings that left hundreds of people unable to vote on May 6.

The Electoral Commission hit out at "inadequate" planning and criticised returning officers for not responding quickly enough when backlogs built up at polling stations.

At least 1,200 people were left queuing in 16 parliamentary constituencies when the polls officially closed at 10pm, according to an interim report from the watchdog.

The chairman of the Electoral Commission, Jenny Watson, said the problems on election night - which have been condemned by politicians from all parties - had been "unacceptable".

"Our review found that some people who arrived before polls closed were unable to vote because returning officers did not have discretion to let them vote after 10pm," she said.

"We are calling for urgent changes to electoral law so that any elector who is entitled to vote and who is queuing at a polling station at the close of poll will be allowed to vote. However, returning officers in the areas affected did not properly plan for, or react to, polling day problems. That is unacceptable. People in these areas were badly let down and have every right to be angry."