The elections watchdog privately warned ministers controversial plans for mandatory voter ID in 2023 were not "fully secure" or "workable".
In a bombshell letter, the Electoral Commission said in the summer it had "fundamental concerns" over the timetable of the multi-million pound policy.
Under existing plans all voters will be forced to show identification at polling stations from 2023 - starting with May's local elections.
Civil liberties campaigners have previously condemned the scheme they warn will erect barriers for members of the public without a form of photo ID.
Letters obtained by Open Democracy show the Electoral Commission raised the alarm on September 20 over the timetable.
Chair John Pullinger warned the then-Levelling Up Secretary Simon Clarke that delays to the legislation "are causing fundamental concerns".
In a frank letter, he said it had prevented election officials from "undertaking the necessary preparatory activity over the summer".
He added: "Crucially, there is now insufficient time for the Commission to test the free voter authority certificate application form and other materials.
"As a result, it is our view that it is no longer possible for the voter ID requirement to be delivered in a way which is fully secure, accessible and workable, ahead of the elections in May 2023."
Mr Pullinger said he had raised the issue with Mr Clarke's predecessor - but "our concerns are yet to be addressed", he added.
Earlier this week a report also raised concerns that election officials across England were "extremely concerned" about the Voter ID proposals.
The study by the Constitution Society said they were worried about ensuring a free and fair vote at the May local elections and could be forced to turn thousands away on polling day.
Nearly half - 45% - of councils surveyed said they were not confident that they can train staff on the new voter ID requirements.
A Government spokesperson told The Mirror: "We cannot be complacent when it comes to ensuring our democracy remains secure.
“Everyone eligible to vote will have the opportunity to do so - 98% of electors already have an accepted form of identification, and any that don’t can apply for a free Voter Card from their council."
They said the Government is "working closely with the Electoral Commission to ensure electors have the right information and training to implement these changes for the May 2023 elections.”
A spokesperson for the Electoral Commission also said: “We are committed to supporting the implementation of the voter ID requirement, but it must be delivered in a way which is accessible, secure and workable.
"We have raised concerns with ministers and department officials that the delays we have seen to date, and the timetable for the introduction of the policy, mean that these important considerations may not be fully met when the new policy is implemented."
They added: “The Government have now brought forward the legislation which sets out how the new process will work in practice.
"This detail has enabled us to start to develop the guidance that electoral administrators will need to implement the changes.
“While the timeframe for introducing the measures before next May’s elections remains tight, we are continuing to support the electoral community and voters to prepare for the change.”
