The democracy watchdog is facing “serious questions” over the handling of its inquiry into the campaign’s referendum spending, according to the head of the pro-Remain group.
Jolyon Maugham QC, director of the Good Law Project, said it was "extraordinary" that the democracy watchdog had taken 18 months to scrutinise whether rules were complied with during the poll.
The commission announced on Monday that it had opened an investigation into spending by the pro-Brexit campaign in the lead-up to the June 2016 vote.
The barrister said it was only the threat of High Court action by his campaign group that spurred the commission into fresh action.
Mr Maugham published contradicting statements he said were from the watchdog that discussed whether it had reopened the inquiry in response to its own review of evidence or as a result of the threat of legal action by his group.
As part of its fresh inquiry, the commission is due to look into whether Vote Leave, the main pro-Brexit campaign during the referendum, exceeded spending limits and entered an incorrect return.
It will also investigate donations made by Vote Leave to student Darren Grimes and Veterans for Britain.
Electoral Commission figures show that Mr Grimes received donations totalling £625,000 from Vote Leave.
This, along with £50,000 from an individual donor, allowed the 23-year-old to spend £675,000 on the BeLeave social media campaign.Veterans for Britain also received a donation worth £100,000 from Vote Leave in the run-up to the vote on June 23 last year.
Vote Leave reported spending totalling almost £6.8 million on the referendum, bringing it close to the £7 million limit for the designated lead campaign. Registered campaigners such as Mr Grimes and Veterans for Britain were permitted to spend £700,000.
Mr Maugham said in a statement posted on Twitter: "It is now 18 months after the referendum vote.
“It is extraordinary that only now is the Electoral Commission taking a serious look at whether the referendum rules were complied with. And only in response to our High Court action."
The barrister said the watchdog had urged the Good Law Project to agree to drop its legal challenge and the group will consider doing so in the coming days.
"But serious questions remain as to whether the Electoral Commission understands its own legal framework," Mr Maugham said.
Announcing its inquiry into Vote Leave, the commission said on Monday: "The opening of this investigation follows a review of previous assessments that the Electoral Commission conducted in February and March 2017 which, at the time, resulted in no further action being taken.
"Since that time, new information has come to light which, when considered alongside the information obtained previously, has given the commission reasonable grounds to suspect an offence may have been committed."
In a separate statement to The Guardian, the commission said its decision to open an investigation "is not based on the legal argument presented by the Good Law Project".
Mr Maugham said an internal document from the commission said: "[The review] was commenced after internal consideration of the papers while responding to a judicial review initiated by the Good Law Project of the decision not to investigate following the original assessments."
The announcement of the fresh inquiry came as anti-EU millionaire Arron Banks, chair of the separate Leave.EU organisation, threatened legal action unless it reveals the sources behind an investigation into the funding of his Brexit campaign, Leave.EU.
His intervention came after the commission said the investigation into Leave.EU was being delayed because the Brexit campaign group had failed to hand over information.
The commission is looking into the alleged undeclared provision of services to Leave.EU by data firm Cambridge Analytica.