A woman on hunger strike to secure the release of her son, the British-Egyptian dissident Alaa Abd el-Fattah, is to protest outside the Foreign Office each day to remind diplomats of his plight.

Laila Soueif is on the 77th day of a hunger strike in which she drinks only tea and which has led her to lose 22kg. The start of her daily protest came as more than 100 MPs and peers wrote to the foreign secretary, David Lammy, expressing their alarm about Abd el-Fattah’s continued imprisonment. The letter is the largest intervention by MPs about his fate since Labour came to power.

They say the Egyptian law is clear that he has served his five-year sentence, since the period of two years in which he was held in detention without trial should have counted as time served, an issue that the government refuses to address.

He was found guilty of sharing a Facebook post about a death in police custody.

Possibly the most admired Egyptian writer to emerge from the Arab spring, Abd el-Fattah should have been released in September, five years after his initial arrest. The cross-party group of MPs and peers includes Conservatives such as Iain Duncan Smith and Lady Warsi, the former party chair who recently resigned the whip. The former Labour Middle East minister Lord Hain is also a signatory.

They write: “Our consular officials are unable to even visit him in prison because the Egyptian government will not recognise his British nationality. We are deeply concerned that any British citizen should be treated in this way and we urge you to use all the diplomatic means at your disposal to secure his release and to allow him to be reunited with his Khaled who lives in Brighton where he attends a special educational needs school.”

The family are furious that a week before David Cameron as foreign secretary met them to discuss how to secure Abd el-Fattah’s release, a £79m UK-Egypt arms deal was approved. Trade between the two countries is worth £4.5bn.

The deal could easily have been stopped by the Foreign Office refusing to grant a licence for the arms exports on the grounds that Egyptians were in breach of UK human rights obligations.

Sitting on a small chair in the cold outside the Foreign Office entrance, Soueif, 68, said: “It is really unfortunate Cameron did not tell us about the arms deals and we only found out about it later.”

Lammy met the family last month and promised to do all he could to secure Abd el-Fattah’s release, but there is concern that Keir Starmer did not raise the case when he briefly met the Egyptian president, Abdel Fatah al-Sisi, at the G20 summit in Brazil.

Our morning email breaks down the key stories of the day, telling you what’s happening and why it matters

after newsletter promotion

Soueif said she was increasingly slow in her movements, but added: “Unless there is a material change in my son’s circumstances, and I do not mean a temporary one, I will continue with this hunger strike. I have only my body left to secure his release.” Her husband was a hugely respected human rights lawyer.

She is chalking on the pavement the number of days her son has remained in prison illegally. Soueif admitted that her son, seen as one of the most perceptive Egyptian writers in the Arab spring, is very down in prison. He is only allowed to see family behind a screen for 20 minutes once a month.