'If the authorities have evidence that Sun Xiaodi and his daughter endangered state security, they should present it in an open and fair trial,' Sharon Hom, executive director of Human Rights in China, said in a statement.

'Instead, they chose RTL -- a non-transparent process of administrative punishment lacking procedural protections -- raising strong suspicions about their handling of these cases,' Hom said.

The rights group said local authorities had 'intensified their monitoring and harassment' of Sun Xiaodi since he won an international Nuclear-Free Future Award in 2006.

Sentences to labour camps are often used to silence local rights activists in China.