Emirati activist Ahmed Mansoor has been on hunger strike for three weeks, Amnesty International said, in protest against "his prison conditions and his unfair trial".
Mansoor was sentenced and jailed "solely for exercising his right to freedom of expression," the UK-based right watchdog added.
Jailed for ten years in 2018 for criticising the government on social media. The 49-year-old previously launched a hunger strike in 2017 as a result of his detention without trial.
His family has not been able to visit him regularly to check on his health, rights groups have previously warned.
Mansoor, an electrical engineer and poet, was among five activists convicted of insulting the UAE's rulers in 2011. They were pardoned the same year.
He was arrested again in March 2017 at his home in the emirate of Ajman, on charges of publishing false information and rumours, promoting a sectarian and hate-inciting agenda, and using social media to "harm national unity and social harmony and damage the country's reputation".
In May 2018 he was sentenced to ten years in prison and fined one million dirhams ($270,000).