The mother of a jailed British Egyptian activist has been hospitalized and is at risk of sudden death, a doctor has said, as her hunger strike to demand her son’s release reached 151 days.

Laila Soueif, the mother of Alaa Abd El Fattah, one of Egypt’s best-known political prisoners, has survived since late September on water, rehydration salts and sugarless tea and coffee to push for his release from a Cairo prison, her family said.

Ms. Soueif, 68, a mathematician and professor who is also a British citizen, started her hunger strike after it became clear that Mr. Abd El Fattah, 43, who had served a five-year sentence, was not going to be released as expected in September.

She told The New York Times last fall that she would not back down in her campaign to pressure the British government to use its diplomatic and economic ties with Egypt to secure his release. “When people ask, ‘What do you think you’re doing?’ I say, ‘I’m creating a crisis,’ ” she said in an interview.

Ms. Soueif lives in Cairo, but has been spending time in Britain throughout her hunger strike and on Monday was admitted to a hospital in London after her blood sugar and blood pressure dropped to dangerously low levels.