Taiwan's former President Chen Shui-bian has been granted a one-month medical parole for treatment, more than four years into his 20-year prison sentence for corruption, the Justice Ministry announced.

Chen suffers from neural degeneration and the medical parole was granted because of worsening condition despite efforts at treatment while he was incarcerated, the Justice Ministry said on Monday in a statement.

"It is believed that only on medical parole can his life and health be ensured, and the degeneration be prevented from worsening," the ministry said.

Chen, 64, whose election in 2000 ended a 50-year monopoly on power by the island's Nationalists, or Kuomintang, and who moved to expand Taiwan's de-facto independence from mainland China, went on trial shortly after the end of his 2000-2008 tenure.

He was convicted in September 2009 of taking more than $12m in embezzlement and bribe-taking in what he denounced as a persecution driven by his political opponents.

Chen reportedly suffered from depression, sleep apnea, heart ailments and neural degeneration while behind bars, and authorities say he attempted suicide.

More than 200 supporters gathered outside the prison in the central city of Taichung where Chen was serving his sentence, waiting to greet him on his release on Monday afternoon.

Chen insists that the charges against him are part of a politically motivated vendetta by the current Kuomintang government, in retaliation for promoting the idea of Taiwan declaring its independence from China.

China claims sovereignty over Taiwan even though the island has ruled itself since their split in 1949 at the end of a civil war. Taiwan has never formally proclaimed independence.