Ministers risked a possible legal challenge to the EU referendum result, after announcing a 48-hour extension to voter registration following a website crash.
Initially, the Cabinet Office suggested only a “short period” of grace would be allowed for people who ran into problems after 10.15pm last night.
The caution came as leading EU-outer Bernard Jenkin raised the prospect of “judicial review of the result” if the deadline was extended for more than a “few hours”.
It is thought that large numbers of those who left registration to the last minute are younger voters – who, polls suggest, are more likely to back the Remain side.
Matt Hancock, the Cabinet Office minister, acknowledged that emergency legislation must be “absolutely watertight” to avert a legal challenge if the referendum result, due early on June 24, is close.
Answering an urgent question in the House of Commons, he told MPs “He (Jenkin) suggested it should be for a short period - and I agree.”
But, just one hour later, Hancock announced the extension would be 48 hours – until midnight on Thursday – despite the website crashing for only around 90 minutes.
He said: “We think it is right to extend to midnight tomorrow to allow people who have not registered time to get the message that registration is still open and get themselves registered.”
The decision followed talks with lawyers and the Electoral Commission – and support from Labour, for legislation to be passed tomorrow.
Under pressure from Labour, the minister defended the “stress testing” carried out on the electoral registration website in the run-up to last night’s midnight deadline.
He revealed that, at its peak, the number of attempted registrations was running at 214,000 an hour – three times the 74,000 an hour just before the deadline for last year’s general election.
Hancock said: “We did, of course, undertake stress testing. The general election was the best comparator – the level of interest was significantly higher than the peak then.
“There was an intense spike after 9.00 pm. The question for the system working is how many people were retrying to apply at one – and that was three times higher.”
There would be a “lessons to be learned” inquiry later, once the issue of how to extend the deadline – and by how long – was resolved, he said.
But Gloria De Piero, for Labour, insisted the chaos was “totally unacceptable” – arguing last night’s surge had been a “predictable rise in traffic”.
Hancock said “alternative sources of data” may be issued in future, to improve voter registration, but said he was not convinced by arguments for auto-registration.