Online intimidation against prominent journalists and the threat of state regulation has left the UK in 40th place in the 2018 World Press Freedom Index which was compiled by Reporters without Borders.
The UK has fallen 18 places since the index was first published in 2002 and is below South Africa (28), Lithuania (36) and Trinidad and Tobago (39).
Rebecca Vincent, UK Bureau Director of non-profit group RSF – which promotes press freedom – said the UK’s place in the 180-country ranking was “embarassing”.
She added: “This is unacceptable for a country that plays an important international standard-setting role when it comes to human rights and fundamental freedoms.
“We must examine the longer-term trend of worrying moves to restrict press freedom and hold the UK Government to account.”
RSF has criticised threats to press freedom in the past year.
This includes proposed reforms to the Official Secrets Act that could have led to journalists being imprisoned for 15 years for obtaining leaked documents.
Britain is also the only country where court action has been initiated over stories about tax avoidance after law firm Appleby started legal proceedings against The Guardian and the BBC.
Laura Kuenssberg had to be assigned security personnel when attending a Labour conference after receiving death threats.
The News Media Association which represents newspapers urged politicians to “protect media freedom and safeguard a vibrant press in the UK”.
A spokeswoman said: We have seen repeated attempts by the House of Lords to hijack legislation such as the current Data Protection Bill, to enforce state-backed press regulation which would have a chilling effect on investigative journalism.
“This is a grave threat to press freedom and could lead to the closure of newspapers.”
Theresa May vowed press freedom “will never change” while she is Prime Minister after privacy campaigner Max Mosley was engulfed in a race-hate row.
Mrs May made the pledge to the Commons after concern was raised about the multi-millionaire’s links to senior politicians and new media regulator Impress.
Former Formula One boss Mr Mosley, 77, was on the back foot over a by-election campaign leaflet from six decades ago which linked non-white immigrants with sexually transmitted diseases, tuberculosis and leprosy.
Speaking at Prime Minister’s Questions, Mrs May said: “Some people will have been surprised to learn of those links with some leading politicians. But can I also say to her that I absolutely agree with her that a free press is very important.
“It underpins our democracy and whatever they say about us, whatever they write about us, actually it’s important that they are able to hold politicians, the powerful, to account and they are able to shine a light in some of the darkest corners of our society.
“As far as I’m concerned while I’m Prime Minister that will never change.”