A full review of the all-postal ballot pilots held in four English regions should now take place amid accusations of electoral fraud says a Labour MP.
14m can were only able to vote by post
Bruce George, who heads a body that monitors elections across Europe, said he was not saying the whole system was "bent" but warned against complacency.
Police probes are underway in the North West after claims of malpractice.
Mr George said: "The complacency gets right up my nose. We need a root and branch look at our electoral system."
The Labour member is president of Parliamentary Assembly of the
Organisation for Security and Cooperation in Europe.
He added: "I am not suggesting the whole system is bent. If there is abuse it is at the
margins. But whole elections can be won or lost in a handful of
Electoral fraud allegations overshadowed the end of campaigning for the European and local elections in the UK.
Claims being investigated by police in Lancashire and Manchester are that some voters have been bullied into handing over their blank ballot papers to be filled in by party canvassers.
Greater Manchester Police are probing malpractice claims while the Lancashire force will question 60 people about 170 proxy vote applications.
Deputy Tory leader Michael Ancram claimed that in one case an employer told his staff he would sack them all if they refused to support Labour.
Meanwhile, in Bradford, there were reports from Asian areas of voters handing over their ballot forms to political canvassers.
The Electoral Commission's chairman Sam Younger has already said he intends to investigate allegations of fraud and intimidation and look at measures to tackle it.
The Conservatives and Lib Dems both opposed Labour's decision to carry out all-postal ballot trials in four regions, and not three as advised by the Electoral Commission.
Deputy Prime Minister John Prescott has insisted any malpractice or illegality will be thoroughly investigated after what were, he stressed, pilot schemes.
He has also argued all-postal ballots - which are being piloted in the North East, North West, East Midlands and Yorkshire and Humber regions - can boost turnout.