On the evening of 26 March, minutes after he took to the stage at London’s Frontline Club, Shahmir Sanni, the Brexit whistleblower, began to weep. The trigger was a reference to No 10 aide Stephen Parkinson’s recent outing of him as gay. “I came out to my mum the day before yesterday,” Sanni told the crammed room as he spoke alongside fellow whistleblower Christopher Wylie. “I hate talking about it, because I get...” As he persisted through tears, Sanni said of his former partner: “He [Parkinson] knew, he knew that I wasn’t out to my mum.”

Sanni, whose intense brown eyes command your attention, lamented that he had been “stripped” of “what was the most important conversation for me to have with my mother and my sisters and my family.” He fears for the safety of his relatives in Pakistan, where homosexuality is illegal. But he is also aggrieved that the story has diverted attention from his defining mission: to expose the alleged illegality of Vote Leave.

The day after Sanni’s appearance, I met him at the Frontline Club, a media institution devoted to “independent journalism”, in Norfolk Place, Paddington. He was joined by Kyle Taylor, the amiable and idealistic director of the Fair Vote Project, a new non-aligned electoral rights campaign. The organisation was launched on 25 April after Sanni’s stunning claim in the Observer: that Vote Leave broke electoral law during the EU referendum campaign by co-ordinating its activities with another group, BeLeave, to which it donated £625,000 in order to avoid breaching the £7m spending limit.

Rather than being provided to BeLeave, the youth organisation that Sanni, now 24, ran with 23-year-old fashion student Darren Grimes, most of the money was allegedly channelled to Aggregate IQ, the Canadian data firm (linked to Cambridge Analytica) that Vote Leave used for its digital advertising.

Sanni told me: “There were emails sent from the head of outreach of Vote Leave [Cleo Watson] to the head of compliance, i.e. the lawyer, saying ‘Shahmir and Darren, the only experience they’ve had of finances is with their student loans, so obviously they’re not the best ones to be handling this money.’ From the get-go this was a process that was guided and directed by Vote Leave staff members, knowing full well that co-ordination between two campaign groups [Vote Leave and BeLeave] isn’t allowed.”

Taylor added: “This massive payment was made to AIQ but then nothing changed. And if nothing changed, then whatever money was paid by BeLeave should have been recorded as part of Vote Leave spending. The body of evidence that suggests this is overwhelming.” (A 54-page dossier of photographs, emails and Facebook chats showing alleged co-ordination has been published.)

Sanni’s allegations bear the imprimatur of three senior barristers (Clare Montgomery QC, Helen Mountfield QC and Ben Silverstone of Matrix Chambers) who concluded that Vote Leave may have “spent huge sums unlawfully”, that there are “grounds to suspect” that campaign director and former Michael Gove aide Dominic Cummings “conspired to break the law” and that Stephen Parkinson and fellow No. 10 aide Cleo Watson may have “conspired with others to commit offences”.

Having assessed the issue twice (and found in favour of Vote Leave), the UK Electoral Commission had already opened a new investigation following a judicial review launched by the Good Law Project. Seventeen days after the Electoral Commission first intervened, Victoria Woodcock, the chief operating officer of Vote Leave, accessed a shared Google drive and removed herself, Cummings and Henry de Zoete (another former Gove aide and Vote Leave’s digital director) from more than 100 files. “We don’t yet know why this was done but it is certainly ambiguous and requires explanation," said the lawyers.

“What I’m hoping is that there is a police investigation into this, particularly the senior members of Vote Leave,” said Sanni. “In my mind, this is me trying to shake everyone: ‘They’re fucking with you, you’re taking it, you’re not questioning it.’”

Vote Leave and its senior staff, it should be said, have denied all allegations. “They know that they cheated,” Sanni replied when I put this to him. “Why aren’t they presenting their own evidence? Why aren’t they showing evidence that there was no coordination?”

Sanni, a polite but forceful character, speaks with the indignance of someone who believed - and still believes - in the Brexit cause. Having emigrated to Britain with his Pakistani family when he was 15, he “never had a European identity”. “The idea of the European Union as this bloc which only benefits European didn’t ever sit well with me.”

He denounced the EU’s free movement policy as discriminatory (“If you’re going to have free movement, it should be free movement for everyone”) and excoriated the Common Agricultural Policy for impoverishing farmers in the developing world.

“It was also about sovereignty, independence, ‘taking back control’, which is what a lot of the Brexiteers voted for,” Sanni continued. “The ironic part is that we’ve lost control of democracy in that process. I was voting Leave to protect our democracy and sovereignty but the opposite has happened, that’s why I’m so impassioned now.”

Sanni has lent his support to the Fair Vote Project, which is campaigning for a re-run of the 2016 referendum (“let’s take back control of our democracy”). Taylor told me: “The issue is too big to have half the country or more than half the country, wonder ‘was that actually the result? Is this the future the country wants?’ That’s first and foremost, let’s be certain, everybody play by the rules.” Rather than a vote on the government’s anticipated Brexit deal, Taylor believes that “on the basis of fairness, it has to be the same vote posed in the same way”.

Like Sanni, Taylor, a US émigré, spoke of how Vote Leave’s alleged actions offended the notion of “fair play” that attracted him to the UK. “I’m an immigrant of choice, everything about being in the UK: none of it’s a right, it’s a privilege. I think when you see it as a privilege it means more, because you have to believe it to stay.”

But unlike Sanni, who would vote Leave in a new referendum, Taylor is a devoted Remainer (he was previously field campaigns director of the pro-EU Best for Britain). “It’s kept the peace,” he said of the EU. But he added of his relationship with Sanni (who he met for the first time last week): “We’ve yet to have a fiery conversation because the issue at hand is the electoral system.”

The Fair Vote Project’s initial funding was provided by ByLine Festival, the event founded by entrepreneur Stephen Colegrave and journalist and author Peter Jukes. “We need to raise £20,000 this week to keep going,” Taylor revealed. “Any donation over £500 will be listed on our website so people know exactly who’s funding this.”

On the evening of 29 March, the group will hold a rally in Parliament Square. As I spoke to Sanni and Taylor, MPs were staging an emergency debate on the allegations against Vote Leave.

Theresa May and Boris Johnson, however, have unreservedly dismissed the claims. Johnson, that reliably trustworthy source, tweeted: “Utterly ludicrous, #VoteLeave won fair & square - and legally.”

May, meanwhile, told MPs: “Whether they voted Leave or Remain, many are frankly tired of the old arguments and the attempts to refight the referendum over the past year.” To the outrage of the Labour benches, she also defended Stephen Parkinson, her political secretary and former Home Office aide, defying demands to sack him (“My political secretary does a very good job”).

“Any statements issued were personal statements,” May said of Parkinson’s outing of Sanni (which first appeared on Dominic Cummings’ blog on 23 March). This, Sanni told me, was “a lie”. “It was an official Downing Street statement that the comms team sent to a major press organisation [the New York Times].”

I put Parkinson’s explanation to Sanni: “That [their relationship] is the capacity in which I gave Shahmir advice and encouragement, and I can understand if the lines became blurred for him, but I am clear that I did not direct the activities of any separate campaign groups.”

Sanni replied; “Number one, I wasn’t his boyfriend during the campaign, it was just a fling. Was Darren [Grimes] his boyfriend too? Was Tom Harwood [the chair of Vote Leave’s student wing] his boyfriend too? Was Cleo Watson my boyfriend?”

His only motive, he insisted, was to establish the truth. “I have no personal gain from this. In fact, I’m losing more than I’m gaining. I’ve lost the opportunity to come out to my mother. I’ve lost, well, I haven’t lost my job - yet.”

Sanni is still employed as the digital campaign manager of the TaxPayers’ Alliance, the libertarian group founded in 2004 by Matthew Elliott - the chief executive of Vote Leave. “John O’Connell, the CEO, has been very professional and supportive,” Sanni said. “His view was ‘I don’t understand what’s going on, I’m going to let you handle it, this is up to the Electoral Commission to decide, I’m not going to get involved. I’m your employer and so I’ll give you leave to sort this out and then we’ll have a meeting.’”

Does Sanni, I asked, fear for his own safety? “To be honest, I don’t care,” he replied. “I would much rather be assaulted in public than outed. But that’s already happened to me, so bring that on, I don’t care.”

In the face of personal and political insecurity, Sanni remained defiant. “The senior members of Vote Leave think that they understand the British public. They think that the British public, particularly Leavers, are stupid enough to believe it’s just another Remain campaign.

“But being a Brexiteer myself I know, from communicating with Leavers from Birmingham to Manchester to London and online constantly, is that if anyone cares about democracy it’s Brexiteers. They [Vote Leave] don’t realise that the British public aren’t stupid and aren’t going to fall for it. That’s why I’m impassioned and encouraged.”