Downing Street has been accused of an “inexcusable” breach of confidence after issuing a statement revealing the sexuality of Vote Leave whistleblower, Shahmir Sanni.

Theresa May’s political secretary on Friday publicly disclosed details of a private relationship with Sanni, just as the Vote Leave whistleblower was preparing to accuse the Brexit campaign of breaking electoral law. 

Steven Parkinson, one of prime minister’s closest advisers, issued a statement in which he said he had been in an 18-month relationship with Sanni, before they split “amicably” in September.

Lawyers acting for Sanni reacted immediately to say the communique was the “first time a Downing Street official statement has been used to out someone”.

Sanni said some members of his family in the UK and in Pakistan were unaware of his sexuality, and that “urgent protective measures” had to be taken to ensure their safety following the statement.

Stonewall, the UK’s leading LGBT+ rights organisation, said in a statement that “the severity of this breach of confidence cannot be underestimated” .

Sanni last week announced he had knowledge of illegal campaign activity while working for Vote Leave. He alleges the campaign funnelled donations through a separate entity to avoid referendum spending limits.

The suggestion has been strongly denied by key figures from the Vote Leave campaign.

Parkinson’s statement was issued on Friday evening via No 10. The Observer newspaper was scheduled to publish Sanni’s full allegations against Vote Leave on Sunday.

But Parkinson later issued a statement on Saturday via his lawyer in a “personal capacity”. 

“My statement to Channel 4 News and The Observer was issued in my personal capacity and was solely a response to the serious and untrue allegations made against me,” it said.

He has since received the support of Theresa May who told the House of Commons on Monday that she recognises “the importance of ensuring that we do recognise that for some being outed as gay is difficult because of their family and circumstances”.

Sanni said that No10’s statement forced him to come out to his parents and take steps to protect his family in Pakistan, where homosexuality is illegal.

Observer journalist Carole Cadwalladr, who has been leading the explosive story on links between Facebook, Cambridge Analytica and the Vote Leave campaign described it as “the story [No 10] tried to kill”.

Sanni told ITV’s Good Morning Britain on Monday: “This weekend has been one of the most awful weekends for me. I didn’t want to talk about [my sexuality] at all because it is not relevant.

“The only reason that this was brought to light was just to make it seem that this was a vendetta, when it is not about me.”

Parkinson’s words also sparked condemnation from former Vote Leave staffers.

Oliver Norgrove tweeted: “I’ve lost complete respect for Stephen Parkinson. His antics are inexcusable and my sympathies are with Shahmir Sanni.”

Stonewall added: “Telling someone about your sexuality or gender identity must always be a personal decision. No person has the right to take that decision away.

“What has happened to Shahmir Sanni is inexcusable. Outing someone can put lives at risk. No LGBT person should ever have to live in fear that someone might tell the world about their sexuality or gender identity before they are ready.

“Only that person will know if they are comfortable and ready to come out.”

Downing Street declined to provide further comment.