Candidates are already preparing court challenges to local election results because of postal voting errors, the Tories said.
Will ballot papers arrive in time?
Nearly a quarter of a million vote papers are having to be reprinted in Stockport in the latest glitch.
The government has dismissed claims that the trials are in chaos and says it is confident they will go smoothly.
But the Tories and Liberal Democrats say the pilots are collapsing and the vote result could be put at risk.
All-postal voting trials are taking place in the North East, North West, Yorkshire and the Humber and the East Midlands for the local and European elections.
The race continues to get ballot forms printed in time and some voters have received the wrong ballot papers.
The Royal Mail had expected to deliver ballot forms this week but some areas are struggling to get them to the postal service in time for the legal deadline of next Tuesday.
Conservative spokesman Bernard Jenkin said the government was to blame for undertaking the trials on such a massive scale in the face of expert advice.
Even if the delivery deadlines are missed, the Electoral Commission says the elections cannot be cancelled unless the government introduces emergency legislation.
But candidates and voters could challenge the results in the courts.
Mr Jenkin told BBC Radio 4's World At One: "I know of candidates at this moment that are briefing counsel on that possibility."
Liberal Democrat chief executive Lord Rennard said it might not be possible to work out how people had intended to vote if there were mix-ups on ballot deliveries.
He warned it was "incredibly difficult and expensive" for candidates to challenge the results in the courts after the elections.
In the North-West, all voters in Stockport are to be issued with new ballot papers after the original papers were found to contain an "unacceptable" amount of mistakes.
Returning officer John Schultz says he is arranging the urgent delivery of the new papers and "rigid" checks will be in place so there is no chance of anybody's vote being counted twice.
It comes after voters in part of Oldham received ballot papers with the wrong ward's candidates' names on.
Other experiencing problems include:
- Sheffield, where 100 ballot papers have had to be reprinted
Bradford, where 500,000 ballot papers have yet to be delivered
The North East, where ballot papers have been delayed in Gateshead, Newcastle, and North and South Tyneside
Gateshead had to switch printers when their contractor's managing director fell ill.
The council believes it can get the papers delivered to voters on Tuesday or Wednesday but because of the delays is allowing voters to put their votes in ballot boxes in public libraries if they wish.
In the East Midlands 27 of the 40 councils involved had yet to deliver ballot packs to the Royal Mail on Thursday.
Constitutional Affairs Minister Chris Leslie on Thursday said there had problems at two printing firms.
But contingency measures were working well and he was confident the 1 June deadline would be met.