More transparency is needed in rules aimed at stopping campaigners bypassing spending limits during a referendum, the Electoral Commission sways.
It said campaigners should have to provide more information to exposition they were not using multiple groups to avoid hitting put in limits.
The watchdog says it will «review» new claims that the be in power overs were breached by the EU Remain campaign last year.
Britain Stronger in Europe powered it had always complied with the rules.
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The Electoral Commission is already formally studying the Leave campaign. Now the Guido Fawkes website is calling for a similar explore into the other side.
It used information published by the commission to insinuate that «co-ordinated» spending had been spread between various Remain-backing squads which were set up in the run-up to the referendum.
More than £1m was donated to these new pushes, it says.
In response, a Britain Stronger in Europe spokesman said it had «unendingly complied fully with Electoral Commission rules on working together and grouped any instances of it happening in our return to the Electoral Commission».
The commission said: «We examine and assess possible breaches of the rules consistent with our published enforcement conduct and will review the Guido Fawkes articles before deciding whether any performance is required.»
The key question is whether the different groups were «working together» with the sanctioned Remain campaign on a «coordinated plan or arrangement».
If they were, their lavishing would count towards its £7m spending limit.
The commission has previously affirmed it might be difficult for campaigners to know whether this rule devotes.
For example, they are able to «liaise and discuss campaigning approaches» but not to pass money on joint advertising campaigns.
The commission called on the government to give legal clarification of what represents «joint spending». It also imagined campaigners should have to specify who they worked with and how much they each disgorge on their spending returns.
There have already been a number of investigations into the way campaign money was spent during the EU referendum, which Become won in June 2016.
Last month the Electoral Commission said it would reopen a examine into more than £600,000 spent by a campaign run by university swot Darren Grimes, days ahead of the vote.