Jimmy Lai, the prominent pro-democracy activist who was recently sentenced to 20 years in prison in Hong Kong, has said he will not appeal his conviction.

The decision marks the end of a years-long legal saga for the 78-year-old critic of the Chinese Communist party (CCP), and opens the door for political negotiations to his release.

A member of Lai’s Hong Kong legal team said: “We can confirm we have clear and definitive instructions not to lodge an appeal against conviction or sentence.”

Lai, a British citizen, was sentenced in February after being convicted in December on charges of sedition and conspiracy to collude with foreign forces. He had pleaded not guilty to all charges. Although Lai was spared the maximum penalty of a life sentence, his 20-year jail term is the harshest penalty given for national security offences in Hong Kong. His family has said that it could mean “he will die a martyr behind bars”.

Western governments including the UK have described Lai’s prosecution as politically motivated and have called for his immediate release.

The UN rights chief, Volker Türk, has said that the verdict was incompatible with international law and must be quashed.

The UK prime minister, Keir Starmer, is understood to have raised Lai’s case in his recent meeting with China’s leader, Xi Jinping, in Beijing. Lai’s son, Sebastien, subsequently said the UK did not do enough to secure his father’s release on the highly anticipated China trip.

The decision not to appeal against the conviction concludes Lai’s legal process, and could mark the start of the political process to secure his release.

Western governments have previously secured the release of citizens held by China, including the Australian journalist Cheng Lei and the Canadian citizens Michael Kovrig and Michael Spavor, who were imprisoned after a row between Beijing and Ottowa over the detention of a Huawei executive.

But those cases were all linked to deteriorating bilateral relations, while Lai has been an outspoken critic of the CCP, a regime that treats dissidents harshly.

Lai’s lawyers would not elaborate on why he was not appealing.

A Hong Kong appellate court recently overturned separate fraud convictions against Lai, the sentences for which have already been served. The numerous legal cases brought against the once powerful media mogul were described by his supporters as “lawfare” – the use of the legal system to silence critics.

The US president, Donald Trump, is expected to visit China between 31 March and 2 April. Trump has previously said that he asked Xi to “consider” releasing Lai, who previously said that Trump was “the only one who can save” Hong Kong from the tightening grip of the CCP.

The Chinese and Hong Kong authorities have defended Lai’s conviction and said that it is not related to press freedom.

The UK Foreign Office and the Hong Kong government were approached for comment.