Pressure is mounting on Britain to follow Canada’s lead and boycott the Commonwealth heads of government meeting (Chogm), due to take place in Sri Lanka in the autumn, after a report by the Sri Lanka Campaign for Peace and Justice accused the authorities of abusing Tamil political prisoners with widespread use of torture, detention without judicial oversight or trial, transferring inmates to prisons around the country without reason, limiting access to families and restricting information to relatives on the condition of prisoners, denying access to those killed in police custody, and restricting or interfering in funeral rites.

The report, Tamil Political Prisoners in Sri Lanka, by N Sivathasan, also discusses the June 2012 riot at Vavuniya jail, when some 200 prisoners protested against three inmates being transferred to a notorious detention facility in Boosa run by the Terrorist Investigation Division, ‘a place notorious for torture’. Police and the army quashed the uprising, followed by severe torture of detainees, with two dying as a result.

One case study describes an artist, identified as MM, who was arrested in 2007. The polio victim claims that after having his toenails extracted and then being electrocuted he was forced to sign a false confession claiming he had helped Tamil Tiger rebels. On the basis of that forced confession, he has been sent to prison in Kandy, far from his home in the north and leaving him practically without contact with his wife or son, who was born months after his arrest.

A UN report last November, which was highly unflattering to the Sri Lanka government, criticised the international body’s own failures in not protecting civilians better in the last stages and aftermath of the war, while another reportby the UN human rights organisation criticised Sri Lanka for not repealing wartime emergency regulations and the Prevention of Terrorism Act, resolving cases of disappearances and ensuring due process for detainees.

Navi Pillay, the UN high commissioner for human rights, said: ‘Unless there is a sea change in the government’s response, which has so far been one of total denial and blanket impunity, a full-fledged international inquiry will clearly be needed.’

She raised her concerns about the protection of witnesses, civil society activists and journalists in Sri Lanka, especially in the wake of ‘calls from certain elements for reprisals in light of the panel’s report’.

David Miliband, former British foreign secretary, added his voice to concerns, writing in the Guardian that ‘For it [CHOGM] to go ahead in Sri Lanka would be a mockery of Commonwealth values and UN authority, and a further invitation for its government to ignore international pleas for decency and accountability.’

Fred Carver, campaign director of the London-based Sri Lanka Campaign for Peace and Justice, said: ‘David Cameron must show leadership by announcing that he will not attend the Commonwealth summit if it takes place in Colombo.’

However, a Foreign Office source told the Observer that they had not ruled out attending the Colombo summit despite evidence of Sri Lanka’s appalling human rights record, saying it was ‘too early to talk about UK attendance’.

When, one wonders, is the right time to discuss it? After the summit has been held perhaps?