Vahid Beheshti’s hunger strike outside the Foreign, Commonwealth and Development Office took a surreal turn on Wednesday – its 70th day – when he attended a royal coronation tea party at Buckingham Palace, arriving by wheelchair and wearing a suit and red tie. He has lost more than 17kg (37lb), or a quarter of his body weight, and he told the Guardian that “my body and joints are now racked in intense pain”. As he left his tent, draped in the Iranian flag and surrounded by flowers, he clutched an envelope containing a letter for the king. After carefully smartening himself up, he was wheeled to a taxi by his wife, Mattie Heaven, a Conservative councillor. At the garden party, he managed to shake hands with Camilla, the Queen Consort, and hand-deliver his letter to the king’s representative. Heaven had received the royal invitation before Beheshti launched his hunger strike and said he was always her plus one and she had not wanted to go on her own. A gentle but deeply determined man, Beheshti has been camped outside the Foreign Office in pursuit of his demand that the government proscribe the Islamic Revolutionary Guards Corps as a threat to British society. He claims that the IRGC has been responsible for 10 death threats in the past seven months. Six other Iranians have pitched tents outside the Foreign Office in an act of solidarity. Beheshti said: “Iranian dissidents and journalists are persecuted and in danger under the Islamic Republic. They shouldn’t also have to fear for their lives in the UK. We have a responsibility to keep them safe from harm, particularly when they are risking everything for freedom.” The foreign secretary, James Cleverly, had yet to stop to talk to him once in the past 70 days, Beheshti said. Cleverly told MPs on Tuesday that he was concerned for Beheshti’s health and urged him to stop. Beheshti said he would only break his strike if he was driven unconscious or the government met his demand, saying his body had grown weaker but his mind was stronger. His doctor, who comes to see him regularly to check his blood pressure and heart, has told him to stop, saying he is causing himself permanent damage. His diet consists of water, coffee and a handful of sugar cubes. Richard Ratcliffe, who endured a 20-day hunger strike outside the Foreign Office to secure the release of his wife, Nazanin Zaghari-Ratcliffe, has said he cannot begin to imagine Beheshti’s state, remembering how his own mind slowed substantially. Heaven worries that ministers are divided over the issue. Beheshti has strong cross-party support for his campaign from MPs – more than 120 have signed a petition backing him – and on Saturday a mood of unity, rare in the Iranian opposition, prevailed as 18 speakers attended a rally to back his cause. Beheshti has met the security minister, Tom Tugendhat, and the Middle East minister, Tariq Ahmad, to press his case. While it is the Home Office that proscribes organisations, it is understood that it is the Foreign Office that opposes such a move in this case, fearing the impact it might have on the British diplomatic mission in Tehran. The Foreign Office says the IRGC is already under sanctions in its entirety, with many individual IRGC members also having sanctions imposed on them, so making them subject to travel bans. Heaven said her husband was not on hunger strike for himself or even for Iran, but for the protection of British values and freedoms. She said Hamas and Hezbollah were banned in the UK, yet the IRGC was the mother of these organisations. The Terrorism Act 2000 requires the Home Office to look at five factors when deciding whether to proscribe an organisation: the nature and scale of an organisation’s activities, the specific threat it poses to the UK, the specific threat it poses to British nationals overseas, the extent of the organisation’s presence in the UK, and the need to support other members of the international community in the global fight against terrorism. Members of proscribed organisations or people who invite support for such groups can be jailed for up to 14 years.