'I have never seen this,' Matas said. 'It is the one and only time China has agreed to give a foreign country access to one of its prisons.'

Lai, a former Fujian businessman, is accused of running a large-scale criminal ring under the cover of his Yuanhua Group. He fled to Canada in 1999, and Beijing formally applied for his extradition in November 2007.

The Chinese offer - included in recent diplomatic documents Matas said he had seen - accedes to giving Canadian embassy and consular officials unmonitored access to Lai while awaiting trial and following any incarceration, assurances of legal representation during the trial, the right to attend the hearing and access to recordings of any pre-hearing interrogations.

Crucially, the deal would also allow Canadian authorities 'reasonable' opportunities to ascertain whether Lai was being tortured in prison - Canada's key grounds for rejecting extradition.

The apparent movement in the case comes after Canada extradited another long-time Chinese fugitive, Zeng Hanlin, in January.

However, it remains unclear whether the offer will lead to any breakthrough in diplomatic wrangling that has gone on for over a decade. For years, China has been trying to convince Canada to deport Lai. One of Beijing's assurances has been that Lai would not be sentenced to death should he return.

Matas said the Chinese government's assurances fell a long way short of convincing him Lai would be safe to face trial in China. The lawyer has filed an extensive submission to the Canada Border Services Agency, an abridged version of which he released to the public.

'The problem with assessing against torture is that torture happens in private,' he said, pointing out that many forms of torture would not leave physical marks. 'How do we know when a person has been tortured? The only way is if the prisoner tells someone.'

Lai's ex-wife Zeng Mingna and his daughter returned to China in 2009 voluntarily, earlier reports said.