An influential group of MPs has urged David Cameron to boycott a Commonwealth summit in Sri Lanka over human rights abuses.
The Foreign Affairs Committee said the decision to hold the Heads of Government Meeting in Colombo next year was "wrong".
The call came in a report on the future of the Commonwealth, which delivered a wider warning that the network's moral authority was being "undermined by the repressive actions of member governments".
The MPs said they were "disturbed" by the ineffectiveness of the mechanisms for upholding the Commonwealth's values, and backed a push to codify them in a charter.
The body's secretariat needs to "sharpen, strengthen and promote its diplomatic performance", according to the report.
"The Commonwealth has appeared less active and less publicly visible in recent years and there is evidence that it is missing opportunities to influence events," the MPs said.
The UK government also came in for criticism over the lack of a "clear and co-ordinated strategy" for its relations with the group. And the report insisted Mr Cameron should take a clear stand on the Heads of Government Meeting.
"We conclude that continuing evidence of serious human rights abuses in Sri Lanka shows that the Commonwealth's decision to hold the 2013 Commonwealth Heads of Government Meeting in Colombo was wrong," the cross-party committee said.
"We are impressed by the clear and forthright stance taken by the Canadian Prime Minister, who has said he would attend the Meeting only if human rights were improved.
"The UK Prime Minister should publicly state his unwillingness to attend the meeting unless he receives convincing and independently-verified evidence of substantial and sustainable improvements in human and political rights in Sri Lanka."