An exhaustive analysis of the campaign’s digital strategy concludes it reached “tens of millions of people” in its last crucial days, after its spending limit had been breached – enough to change the outcome.
The evidence will be put to the High Court on Friday, in a landmark case that is poised to rule within weeks whether the referendum result should be declared void because the law was broken.
Professor Philip Howard, director of the Oxford Internet Institute, at the university, said: “My professional opinion is that it is very likely that the excessive spending by Vote Leave altered the result of the referendum.
“A swing of just 634,751 people would have been enough to secure victory for Remain.
“Given the scale of the online advertising achieved with the excess spending, combined with conservative estimates on voter modelling, I estimate that Vote Leave converted the voting intentions of over 800,000 voters in the final days of the campaign as a result of the overspend.”
The conclusion came as Theresa May scrambled to find a concession she could give to rebel Tories to persuade them to back her deal, as she appeared to be veering towards a heavy defeat in next Tuesday’s landmark vote.
Professor Howard’s report is based on separate research which found that 20-30 per cent of people decided how to vote within a week of polling day, with half of these doing so on election day itself.
If, as he has concluded, Vote Leave’s Facebook adverts reached tens of millions of people after they should have stopped, they influenced huge numbers of voting decisions.
Now the campaigners will ask the High Court for permission to use the report as evidence, when a potentially explosive legal challenge gets underway on Friday.
The Independent revealed last month that lawyers expect the judges will fast-track the case to a full hearing because of the prime minister’s refusal to act on the growing evidence of illegality in the 2016 referendum campaign.
Lawyers describe that failure as “absolutely extraordinary” – given the National Crime Agency’s (NCA) separate investigation into suspicions of “multiple” criminal offences committed by Arron Banks and the Leave.EU campaign.
The case is expected to move to a full hearing and a ruling within weeks of opening, with the clock ticking on the UK’s departure from the EU next March.
The government is set to deploy Sir James Eadie QC – the barrister who led the unsuccessful battle for the government to trigger Article 50 without parliament’s consent – in a sign of the case’s importance.
Professor Howard based his study on material disclosed by Facebook to the Commons committee which investigated Vote Leave’s overspending, as well “accounts written by campaign insiders”.
It points out that the Remain campaign was forced to stop its digital advertising on the last day of the June 2016 campaign because it had reached its spending limit.
In contrast, Vote Leave carried on, despite busting its limit two days before the vote – and was later found by the Electoral Commission to have broken the law.
Friday’s case has been brought by a group of British expats in other EU countries, called UK in EU Challenge, which argues lawbreaking during the campaign means the result cannot be considered the “will of the people”.
The professor was asked to provide an expert’s report which Croft Solicitors, acting on behalf of the claimants, will seek to use as evidence in its case for a substantive hearing.
Carole-Anne Richards, one of the claimants, said the public had the right to expect the referendum to have been carried out “lawfully, free from corrupt and illegal practices”, adding: “This is clearly not the case here.
“We are very grateful to Professor Howard for his work in looking at Vote Leave Facebook advertising overspend and correlating that with voter behaviour.”
The group will argue that Brexit must be declared void and the notification of Article 50 quashed, because “various criminal offences may have been committed”.
It had been expected that any ruling would await the conclusions of the NCA probe, but it is now thought it will be fast-tracked – with less than four months until Brexit day.
Mr Banks could be given a two-year prison sentence if he is found to have “knowingly concealed” that an Isle of Man company was the source of up to £8m for Leave.EU – which would have been outlawed as a foreign donation.
Croft Solicitors wrote to the prime minister on 6 November asking her to “reconsider her refusal to take any action” over the lawbreaking exposed during the campaign and the announcement of the NCA investigation.
Downing Street declined to respond to The Independent’s request for a response about why there has been no wider investigation into alleged illegality.
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