Sri Lankan editors have written to David Cameron urging him to put a stop to the royal charter on newspaper regulation, claiming it will serve as a blueprint for those who want to control the press around the world.

The group appealed to the prime minister to protect Britain's reputation as a "shining example to be followed" before his attendance at the Commonwealth heads of government meeting in Sri Lanka, which has been accused of intimidating journalists and restricting freedom of speech.

They said Cameron should not underestimate "the concern that many of us feel in Sri Lanka and, we understand, across the Commonwealth because of the actions of the British government", after the Queen approved a royal charter that would oversee a new independent press regulator.

Cameron has agreed the new structure for press regulation with the Liberal Democrats and Labour after the Leveson report on media ethics recommended greater oversight of the press following the phone-hacking scandal. However, many newspaper groups in the UK are resisting the new controls.

The Sri Lankan editors wrote: "Since countries like Sri Lanka regained their independence, and the advent of the Commonwealth, Britain acted as a beacon of freedom to the oppressed across the world. The actions of your government now send out a different blueprint to those seeking to control the press."

They said Cameron should think about how his actions looked to those regimes that Britain condemned for restricting freedom of speech. "When looking into the eyes of those your government believe have veered from the path of democracy, British prime ministers and foreign secretaries alike will need to be able to speak with conviction and surety," their letter said.

The group said the "fundamental bedrock of any democracy must be a free and independent press totally out of the reach of the politicians and apparatus of the state that it reports on", but this appeared to be under threat in Britain. "Democracy across the world shudders at what is happening in the United Kingdom, and Britain unfortunately stands smaller in the world as a result of your government's actions," they said.

Asked about the letter, Cameron's official spokesman said: "With regard to the royal charter, the prime minister would not share their view. When it comes to robust engagement with the Sri Lankan government, is freedom of expression one of the very important issues we will be raising? Absolutely."

The World Association of Newspapers and News Publishers (Wan-Ifra) has announced it will lead an international delegation to the UK in January to examine concerns about the royal charter and the government's condemnation of the Guardian's reporting on leaks by the US whistleblower Edward Snowden.

Vincent Peyregne, the Wan-Ifra chief executive, said: "It is rather difficult for the United Kingdom to lecture Sri Lanka and others about their press freedom record, when its own actions result in such widespread international condemnation.

"We are concerned that these actions not only seriously damage the United Kingdom's historic international reputation as a staunch defender of press freedom, but provide encouragement to non-democratic regimes to justify their own repressive actions."

It will be the association's first official delegation to Britain, after previous trips to countries such as Ethiopia, Libya, Yemen and Burma.