Archbishop Desmond Tutu condemned the decision on Tuesday to return democracy icon Aung San Suu Kyi to house arrest in Myanmar, saying her trial was illegal and must not be accepted.

In a statement issued in London, the Nobel laureate said the decision by the ruling military junta to commute Aung San Suu Kyi's sentence of hard labour to 18 months under house arrest was "a manipulation of an illegal process".

"This decision is wrong. We must not allow the government of Burma (Myanmar) to convince the world that they are making concessions in relation to Aung San Suu Kyi by returning her to house arrest instead of prison," Tutu said.

"This is not a concession ? it is a manipulation of an illegal process. It must not be accepted by any government, Asean (Association of Southeast Asian Nations), the EU or the UN."

Suu Kyi was found guilty by a court on Tuesday for breaching the terms of her house arrest, after an American man swam uninvited to her lakeside home in May.