Israeli security agents held a Palestinian patient for three weeks without charge, interrogated him repeatedly and offered access to hospital care if he agreed to become an informant, the Guardian has learned.

The treatment of Abd al-Karim al-Atal, 28, is the latest in a series of cases over the past two years in which patients from Gaza referred for hospital treatment in Israel have been held without charge and pressed to become Israeli collaborators, human rights groups say.

Atal, who is losing his sight, is still waiting for a permit to travel from his home in the Jabaliya refugee camp, in Gaza, to an eye hospital in east Jerusalem for a cornea transplant operation now scheduled for tomorrow.

Physicians for Human Rights, a leading Israeli rights group, says the pressure exerted on these patients amounts to coercion, which is illegal under the fourth Geneva convention, and may even constitute a breach of the UN convention against torture. It says around one in five Gazans who apply for permits to enter Israel for medical care are now submitted to detailed interrogations.

B'Tselem, another human rights group, says Israeli security agents "exploited the questionings to exert inappropriate pressure on ill persons, with the aim of forcing them to collaborate with the agency".

Israel says such questioning is a necessary security measure to prevent terrorist attacks and says that 5,000 people – patients and their relatives – have been allowed out of Gaza for medical reasons this year. But Ami Gil, of Physicians for Human Rights, said while initial screening of patients referred for treatment in Israel was a legitimate security consideration, the problem lay in the pressure put on patients under interrogation.

"There is a screening process to prevent a security threat and another to pressure patients to gather intelligence information that has nothing to do with their own case or background," he said. "That is not about screening. It is about gathering information for intelligence purposes."

Atal has a referral from Gazan health officials supported by the St John Eye Hospital in east Jerusalem, which states that he needs a penetrating keratoplasty – a cornea implant. In the west that would be routine, but no hospital in Gaza can perform the operation.

He applied for a permit to enter Israel and in early September was called to the Erez crossing which leads into Israel. He was blindfolded and handcuffed for a time. An Arabic-speaking Israeli security officer accused him of falsifying his medical papers. In fact, his vision is so poor he can barely see out of his left eye and has limited vision in his right. Atal, a former member of Gaza's Fatah-led police force, was asked to give detailed information about his five brothers and an uncle living in Egypt. He was accused of involvement in the Al-Aqsa Martyrs Brigade, the military wing of the Fatah movement.

"They said if I accepted and gave them information they would allow me to return home and to get a permit in future. If I refused they said they would arrest me," he said. When he said he had no information to give he was taken to a detention centre in the nearby city of Ashkelon.

He was photographed, fingerprinted and then held for 19 days alone in a cell with no windows. He was interrogated for hours at a time while seated on a small chair with his hands cuffed behind his back underneath an air conditioner pouring out cold air.

Eventually he was questioned while attached to what he was told was a polygraph machine. He was asked about his relatives, about his neighbours and about any Hamas leaders he knew. Again he was accused of involvement in militant groups. "I looked around and said: 'Are you talking to me? I can hardly recognise people in the street?'

"They said if I collaborated with them it would be a good thing for the Palestinians, that it would help them target Hamas leaders, not accidentally kill civilians," he said. "They said I should call them and tell them about my neighbourhood: who is living where, is anyone from Hamas there. They said I would get a permit to enter Israel in return. They offered money, they said I would be allowed to travel abroad."

Israeli officials deny that entry to Israel for medical reasons is conditional on patients becoming informants but they say security is an issue. In June 2005 a female suicide bomber wearing an explosives belt tried to cross through Erez and Palestinians have used false medical papers in the past. Last year a government official wrote to Physicians for Human Rights saying the questioning was "intended to evaluate the degree of danger posed by the applicant".

"For us it is not only a legal issue, but a very basic moral issue," said Gil. "We are talking about patients here."