A prominent Leave campaigner is considering launching a legal challenge after the government extended the EU referendum voter registration deadline.
Leave.EU founder Arron Banks said there were grounds for a judicial review of the "unconstitutional" move.
It comes after the government website for registering voters failed just before Tuesday's original deadline.
A Britain Stronger in Europe source said Mr Banks was entitled to spend his money as he wished.
The government has pushed through emergency legislation to allow people to register until 23:59 BST on Thursday.
The extension to the deadline covers everywhere apart from Northern Ireland, where the online system was not in use.
The BBC understands that if there is another glitch before the new deadline the registration period will not be extended again.
The site's capacity has been expanded but anyone wanting to register to vote - and anyone who voted in the May elections will not need to unless they have moved home - is still being advised not to risk leaving it until close to midnight.
Cabinet Office minister Oliver Letwin said in the hour leading up to the crash there had been 214,000 registration applications.
Tuesday's spike was "three times as intense a spike as occurred before the general election" he said, and it would now take a spike "six times as large" to cause the site to fail again, he told MPs.
According to the official government website, there were 242,000 applications to register to vote on Wednesday, the second highest total since mid-May when a registration campaign was launched. Of these, 135,600 were from people aged under 35.
Traffic to the website on Thursday seems to be lower, with about 2,000 to 2,500 people using it at any one time.
'All legal options'
In an interview for BBC Radio 4's Today programme, Mr Banks, an insurance millionaire, told political correspondent Ross Hawkins: "We've got lawyers that are looking at it at the moment.
"They are tending to say it's unconstitutional because once you've set the rules you can't really change it halfway through, and Parliament really shouldn't be doing this."
In a statement Mr Banks said: "We believe it is unconstitutional at best and have been advised that with legitimate cause we could challenge this extension.
"We are therefore considering all available legal options with our legal team, with a view to potentially launching a judicial review now and after the outcome of the referendum on 23 June."
There were calls to extend the deadline from across the political divide after the online problems emerged, although some Conservative backbench MPs have criticised the move.
BBC political correspondent Ross Hawkins said many Leave campaigners see the deadline extension as a "fix" because they think people signing up late will be younger and therefore more likely to support the EU.
The official leave campaign - Vote Leave, in which Mr Banks plays no part - has said the government is trying to register as many likely Remain voters as possible, but stopped short of suggesting that it would consult lawyers.
A Britain Stronger in Europe source said: "Mr Banks is entitled to spend his money as he wishes. We'll keep pointing out Britain is stronger, safer and better off in the EU - and leaving is a leap in the dark."
The chances of any judicial review succeeding are impossible to assess until the legislation on which MPs will vote on Thursday has been published and studied.
Leave.EU threatened, but did not bring, a judicial review against the Electoral Commission's decision to make Vote Leave the designated lead out campaign.