Behairy, who began his hunger strike in March after five years in custody, has been refused pen and paper, subjected to round-the-clock cell lighting and had visitation rights slashed to just 20 minutes a month, the Egyptian Commission for Rights and Freedom said.
The NGO called for his immediate release.
In a letter smuggled out of Badr prison, where he is awaiting trial, Behairy vowed: "I will continue this strike until I regain my freedom, alive or not," the rights group said.
Behairy was sentenced to three years in prison in 2018 for "insulting the military establishment" after publishing a collection of satirical poetry and writing song lyrics critical of the government.
In 2021, when he was scheduled for release, the public prosecution levelled new charges against him of "joining a terrorist group and spreading false news" - charges often leveled against dissidents in Egypt.
Human rights groups have repeatedly criticized what they call a "revolving door" justice system in Egypt, in which prisoners are frequently kept in jail to face new charges instead of being released.
They say around 60,000 political prisoners are languishing in Egyptian jails, many facing brutal conditions in overcrowded cells.
There have been a number of hunger strikes in recent years, including one by jailed British-Egyptian activist Alaa Abdel Fattah which made international headlines during the COP27 climate summit in Egypt last year.
Abdel Fattah eventually gave up his protest in November after seven months of eating no more than 100 calories a day, his family said.
The government announced a new "national human rights strategy" in late 2021, launching a "national dialogue" and releasing hundreds of hundreds of detainees.
But although around 1,000 prisoners have been freed over the past year, almost three times that number have been newly arrested, according to Egyptian rights groups.