Campaigners have urged David Cameron to raise gay rights at a summit of conference leaders, warning abuses across its member countries are being swept under the carpet.
Same sex relationships are illegal in 41 of the 53 nations invited to the gathering in Sri Lanka - making up more than half of the countries in the world where consensual gay sex remains a criminal offence.
But the Kaleidoscope Trust said the issue would once again be "the elephant in the room" as it was absent from the formal agenda for the Commonwealth Heads of Government Meeting (Chogm).
It has produced a report of eye witness reports of abuses and demands a repeal of all bans - and a halt to prosecutions while it is done as well as a commitment to discuss the issue at the meeting.
Spokesman Jamie Elliott said Mr Cameron had won respect for pushing through the legalisation of same-sex marriage in the UK and being pro-active in seeking to promote gay rights more widely.
It was essential that he did not hesitate to maintain that stance when he meets with the leaders in Colombo later in the week, he said.
Former head of human rights for the Commonwealth Secretariat Purna Sen said: "Once again the human rights of LGBTI (lesbian, gay, bisexual, trans and intersex) people are the elephant in the room.
"There seems little doubt that the Sri Lanka Chogm of 2013 will once again fail to address the legitimate demand for equal rights and dignity for all," she wrote in the report.
"By doing so it will call into question the Commonwealth's credibility and relevance. The people whose voices are contained in this report may be ignored but they won't go away.
"Governments must heed them, must meet with them and must embrace them as full and equal members of society.
"Anything less will render the fine sounding words opposing 'all forms of discrimination' meaningless and condemn the Commonwealth as impotent in the face of injustice."
Sir Shridath Ramphal, who served as Commonwealth Secretary-General from 1975-1990, wrote in his foreword that anti-gay laws were "a relic of empire that has no place in a modern Commonwealth".
A Downing Street spokeswoman said: "The Prime Minister and Foreign Secretary will make clear their concerns about the human rights situation in Sri Lanka when they visit Colombo this week.
"The Commonwealth Charter, agreed by all Commonwealth members, explicitly states that we are opposed to all forms of discrimination and it is important that all members live up these values.
"That is the message that we will be taking to the summit."
Shadow foreign secretary Douglas Alexander said: "Today's report from the Kaleidoscope Trust highlights the ongoing concern about human rights - and in particular the rights of the LGBT community - within Sri Lanka.
"New allegations in this report of abuses and intimidation of LGBT citizens are a further warning that President Rajapaksa's government has not made the progress ahead of this Commonwealth summit that we all wanted to see.
"As David Cameron departs for this week's Commonwealth summit, the evidence that Sri Lanka is heading in the wrong direction is mounting, which is why Labour has called on the Prime Minister to use what leverage he has in the run up to the summit to pressure the Sri Lankan government to change their approach on human rights."