OCCUPIED JERUSALEM: Israel is holding 309 Palestinians in prison without charge, a sharp increase from last year, an Israeli human rights group said on Tuesday, calling for the prisoners to be charged or released.

B'Tselem said figures received from the Israel Prison Service (IPS) showed there were 309 Palestinians held under so-called administrative detention orders in January 2012, up from 219 in January 2011.

The group said 80 of those being held had been detained for between six months and a year, another 88 had been held for between one and two years, and 16 of them had been behind bars for between two and four-and-a-half years.

One man has been held in administrative detention for over five years, the group said.

The figures were released as the process of administrative detention receives new attention because of the case of Khader Adnan, a Palestinian being detained without trial who has been on hunger strike for 66 days.

Adnan began refusing food after he was arrested on December 17, and says he is protesting his detention without trial as well as alleged mistreatment during his interrogation.

Israel has not made public any charges against Adnan, who had served as a spokesman for Islamic Jihad.

Evidence submitted to a military court in support of Adnan's administrative detention is kept secret, with even his lawyer denied access to it.

Israel says administrative detention is necessary to keep dangerous individuals behind bars, but B'Tselem said the way the Jewish state was using the procedure was "patently illegal."

"Individuals are not told the reason for their detention or the specific allegations against them... Most of the material submitted by the prosecution is classified and not shown to the detainee or his attorney," B'Tselem said.

"Since the detainees do not know the evidence against them, they are unable to refute it. The detainees also do not know when they will be released."

Current rules allow an Israeli court to approve an administrative detention order of up to six months at a time, although detainees are able to appeal against it.

The state must apply to renew the order every six months, but a detainee can be held indefinitely if a court is willing to keep renewing the order.

Adnan, who received a four-month administrative detention order, has unsuccessfully appealed his detention already and is expected to challenge the order before Israel's Supreme Court on Tuesday.