Giving evidence to MPs, the former Cambridge Analytica employee Chris Wylie said it was "unreasonable to come to any other conclusion" about why independent campaign groups hired the same Canadian data company "other than this must be coordination".
Mr Wylie and other whistle blowers have claimed Vote Leave coordinated with other campaign groups in order to avoid a £7m legal limit on election spending, in breach of electoral law - something the official Leave campaign and its leaders deny.
Attention has focused on £675,000 that was given by Vote Leave to BeLeave, a group set up by 22 year-old fashion student Darren Grimes.
Almost all the money was then spent on hiring AggregateIQ (AIQ) to target advertising at web users. Vote Leave also spent 40% of its budget with AIQ.
A further £32,000 was paid to AIQ by the DUP after receiving £425,000 from the Unionist organisation the Constitutional Research Council, a firm chaired by Richard Cook, a former vice-chairman of the Scottish Tories.
"All of these companies somehow, for some reason, all decided to use AIQ."
He questioned how AIQ, which had little public profile, became the service provider to each of the organisations.
"When you look at the accumulation of evidence, I think it would be completely unreasonable to come to any other conclusion other than this must be coordination, this must be a common purpose plan," he said.
Mr Wylie claimed he went to AIQ's head office and was told: "It was totally illegal." He told the committee: "I think it is completely reasonable to say there could have been a different outcome of the referendum had there not been, in my view, cheating."
Mr Wylie told MPs he had met Vote Leave's campaign director, Dominic Cummings, in November 2015, at a point when it was clear that the eurosceptic group had no database of its own.
"Very shortly after that meeting, they then engage AIQ," he said, adding that his speculation was that the company was chosen because it would have "looked odd" if both Vote Leave and the separate Leave.eu Brexit campaign were both working with CA.
Vote Leave had got "the next best thing" to Cambridge Analytica by hiring "a company that can do virtually everything that CA can do but with a different billing name", he said.
The Canadian company has been accused of mounting disinformation campaigns and spreading threatening election campaign videos in places like Nigeria. Commenting on their involvement in the Brexit campaign, Mr Wylie told the committee: "They found it amusing. You have to remember this is a company that's gone around the world and undermined democratic institutions in all kinds of countries.
"They couldn't care less if their work is compliant because they like to win."
Mr Wylie added: "For me it makes me so angry because a lot of people supported Leave because they believe in the application of British law and British sovereignty and to irrevocably alter the constitutional settlement of this country on the basis of fraud is a mutilation of the constitutional settlement of this country."
The whistle blower was also asked whether Cambridge Analytica, which is under investigation in the UK and US over whether it used harvested Facebook data to target campaign messages during the American presidential election and the Brexit referendum, had worked on the 2014 Scottish independence referendum.
Mr Wylie told MPs: "I know Alexander [Nix, the suspended CEO of Cambridge Analytica pitched for work in relation to the Scottish independence referendum but I'm very fuzzy on the details on what side that was for and what the actual pitch was"