Conservative peer Lord Naseby has suggested the UK government is not abiding by David Cameron’s promised timetable for co-operating with the UN to set up an independent inquiry into claims the Sri Lankan government carried out war crimes.

He asked why the UK government is “already working to influence the United Nations Human Rights Council” when the prime minister said in November that the UK would allow Sri Lanka until March to begin its own investigations into alleged war crimes before taking steps through the UN.

“Conclusive evidence” had emerged, Lord Naseby asserted, that so-called witnesses to alleged war crimes in a documentary by Channel 4 were “fully paid-up members of the Tamil Tigers”.

He also called for the publication of the UK military attache in Colombo’s dispatches about the final stages of the 26-year civil war, during his contribution at question time on 26 February 2014.

Foreign Office Minister Baroness Warsi told the House she was aware of new material concerning the war, which lasted from 1983 to 2009.

But she defended the government’s discussion of the proposed inquiry with UNHRC members since, she said, “we have yet to see a meaningful, time-bound, independent, domestic-led political process with clear milestones”.

Labour spokesman Lord Bach confirmed the opposition’s support for the government’s approach and urged the prime minister to be “true to his word on this” and work closely with allies to secure an inquiry at the “earliest possible time”.

Lib Dem Lord Avebury regretted that “no progress has been made” over the past five years, but went on to congratulate Mr Cameron for his commitment and asked whether enough UN members would vote for a resolution to hold an inquiry.

The minister said it would be “wrong for me to predict at this stage” how members would vote and assured members the UK government would “continue to work incredibly hard” for a resolution, BBC reports.