Israeli police have arrested Mordechai Vanunu, a technician who spent 18 years in prison for revealing details of Israel's clandestine nuclear programme.
He is being held on suspicion that he met foreigners, violating conditions of his 2004 release from jail, police say.
At a Jerusalem court hearing, Mr Vanunu was placed under house arrest for three days until the case proceeds.
His lawyer said his arrest was because of his relationship with his Norwegian girlfriend, not for revealing secrets.
"Vanunu was arrested [for] a relationship between a man and a woman, with a Norwegian citizen," Avigdor Feldman told reporters.
"She is not interested in nuclear business - she's interested in Mordechai Vanunu [and he] is probably interested in her."
But police spokesman Micky Rosenfeld said Mr Vanunu had met "a number of foreigners", something he had been banned from doing.
From the data Mr Vanunu leaked to a UK newspaper in 1986, experts concluded that Israel had nuclear arms. Israel neither confirms nor denies this.
After his release from prison in 2004, the Israeli authorities banned Mr Vanunu from speaking to foreign media and travelling abroad.
They said he could divulge more classified information about Israel's Dimona nuclear plant, where he had worked before the arrest.
Mr Vanunu - an anti-nuclear campaigner - has rejected the claim, saying he only wants to be free to leave Israel.
In 2007, Mr Vanunu, a Jewish convert to Christianity, was sentenced to six months in prison for breaking the conditions of his parole.