The handful of teens and twenty-somethings who made up BeLeave seemed an unlikely group to attract the attention of Britain’s most powerful Brexiters, but long before they got a large, controversial donation from Vote Leave they had made their way to the heart of the official campaign.

All politically inexperienced volunteers, they were welcomed into the team backed by Michael Gove and Boris Johnson, mostly because of the demographic they promised to reach.

It was Darren Grimes, then a 23-year-old fashion student from County Durham, who set up the youth group. A Liberal Democrat member with a talent for graphic design and social media marketing, he had an approach that was rare among the groups campaigning for Brexit.

BeLeave messages targeted a group generally inclined towards Remain – young, diverse and progressive voters – who the senior campaigners at the official Vote Leave campaign thought would be both hard to win over and critical to any victory. “Vote Leave understood that they couldn’t win the referendum if they specifically targeted angry Ukippers. They knew that they needed to target young liberals,” said the group’s treasurer turned whistleblower Shahmir Sanni.

“They needed to target Green party members that didn’t like the EU’s environmental laws, or liberal Eurosceptics like me that did hold fiscally conservative values but were socially liberal and understood the EU didn’t support everyone, that it only benefited Europeans.”

When Sanni joined the BeLeave team, it had already become extraordinarily successful in appealing to liberal and leftwing supporters on Facebook and other social media, using videos and other messaging that often appeared to be the exact opposite of the largest Vote Leave and LeaveEU campaigns.

Instead of bleak scaremongering about immigration, most notoriously Nigel Farage’s “Breaking Point” poster, their adverts were pro-migration, feminist and witty. One campaign questioned why holders of EU passports could come freely to the UK while Commonwealth citizens could not. Others attacked the EU for policies it claimed “have driven African farmers into poverty”, and made it impossible for Britain to abolish the “tampon tax”. A fourth warned that EU penalties could make Netflix streaming more expensive.

Their positions stood out, even amid the patchwork of pro-Brexit groups, and Vote Leave realised BeLeave could prove useful as it battled for designation as the official campaign against the Arron Banks- and Farage-backed Leave.EU. Needing to show a broad spectrum of support, it listed the youth group in its application, among a range of independent “outreach groups” claimed to be backing Vote Leave.

Yet at the time BeLeave worked out of Vote Leave headquarters, under its guidance and support. Although it had its own website, it appears to have been co-ordinated and paid for by the Vote Leave head of outreach Cleo Watson. “You deserve a brilliant site and this is so annoying,” Watson, now a political adviser to the prime minister, wrote to Grimes when development of the group’s site hit a glitch. “Will troubleshoot this asap.”

Watson said: “I absolutely deny the claims being levelled against me, including the allegation that I co-ordinated and paid for the BeLeave website”. She added: “To imply being supportive of their work was to have any kind of control over their activities is absolutely untrue and deliberately misleading.”

But in the final weeks before the referendum, as Vote Leave neared its spending limit of £7m, senior directors suggested to BeLeave that, if they set up as a separate campaign, they would receive a donation to spend on their own advertisements.

Preparing for meetings with prospective donors – and unaware that they were working beside the people who were planning to support them – the team drew up a pitch for funds. Even their highest hopes were well below the eventual donation. “On our proposal we wrote £10,000 would get you this much engagement on Facebook, and if you gave us £100,000 this is how much engagement we’d get,” Sanni said. “We were putting it there just in case.”

They pointed out that their Facebook posts were engaging more people – to share, comment or like – than most paid advertisements by political campaigns. One video reached 41,000 people in one day without any paid promotion. “With funding, we could triple this amount,” their pitch said.

Shortly afterwards Vote Leave drew up legal documents allowing BeLeave to create a bank account so that it could accept donations of its own. “Following our discussion I attach a typed-up first draft of the constitution,” Vote Leave’s legal director said in an email to Grimes in May, with Watson copied in.

He told Sanni, BeLeave’s secretary and treasurer, to set up a bank account. Then Vote Leave revealed to the astonished young team that they would be getting a donation of more than £600,000. “To be at that age and to find out that my organisation, that the work that I’ve done, has received funding of almost £1m, I was elated,” Sanni said. “It was amazing.”

The donation never made it into the bank account. The money would be passed directly to AggregateIQ – Vote Leave’s data analytics firm – in its name, without ever coming under even nominal control of BeLeave.

“The first thing I said to Darren was, ‘Well, OK, can I get my train tickets paid for?’,” remembers Sanni. But Grimes made clear that he had no control over even small amounts.

The BeLeave team were allegedly not warned of potential implications. “If I am being advised by ... a compliance lawyer for Vote Leave, I have no reason to think anything is wrong,” Sanni said. Vote Leave has denied any suggestion it co-ordinated with BeLeave.

When the size of the donation drew scrutiny from the Electoral Commission, Grimes said he found AIQ on the internet and was so impressed he decided to blow his budget on its marketing. But cached data shows that, at the time, AIQ did not have a website. AIQ says it had a website since it was founded in 2013.

Grimes knew about the firm because of its work with Vote Leave, whose director and other employees sat at desks near his own in the campaign HQ, where BeLeave’s work, whether in messaging or as a conduit for funds, was highly appreciated. “Everyone knew who Darren was,” Sanni says. “Everybody congratulated us and knew what role we’d played. He knew exactly how important we’d been. He’s a close friend of Dom Cummings [the Vote Leave strategist]. Of course he knew. Boris Johnson knew. Everybody knew.”

Grimes denied to the Observer that there was any collaboration with Vote Leave on campaign material or spending.