Tensions over the decision to allow US authors to enter the Man Booker prize have flared up yet again, with 30 publishers signing a letter urging the prize organisers to reverse the change, or risk a “homogenised literary future”.

The letter, which was intended to be private and has been seen by the Guardian, argues that the rule change to allow any writer writing in English and published in the UK to enter has restricted the diversity of the prize and led to the domination of American authors since it came into effect in 2014. Previously, the prize only allowed citizens from Commonwealth countries and the Republic of Ireland to enter.

“The rule change, which presumably had the intention of making the prize more global, has in fact made it less so, by allowing the dominance of Anglo-American writers at the expense of others; and risks turning the prize, which was once a brilliant mechanism for bringing the world’s English-language writers to the attention of the world’s biggest English-language market, into one that is no longer serving the readers in that market ... [It] will therefore be increasingly ignored,” the letter claims.

Mark Richards, publisher at John Murray, confirmed that the letter had not yet been sent to the trustees of the Booker Foundation. “There is a letter circulating that we hope, once sent, will start a constructive discussion with the Booker Foundation,” he said.

Responses to the rule change have been varied since it came in. When it was first announced, author Philip Hensher wrote: “I don’t think I’ve ever heard so many novelists say, as over the last two or three days, ‘Well, we might as well just give up, then.’” Julian Barnes, who won the Booker in 2011, branded the change “straightforwardly daft”; broadcaster Melvyn Bragg said the prize might lose its distinctiveness; and Susan Hill, who judged the prize twice and was shortlisted once, wrote: “Not sure I can see a reason for this. Why can’t we have a prize of our own?”

Writers in the UK don't have any reason to be paranoiac about competition from the US. They should welcome the challenge Alastair Niven, 2014 Man Booker judge

But others have been supportive. Nobel winner Kazuo Ishiguro, who won the 1989 prize, said “the world has changed and it no longer makes sense to split up the writing world in this way”. Alastair Niven, a prize judge in 2014, said on Friday: “I don’t think that writers in this country have any reason to be paranoiac or timid about competition from the US. They should welcome the challenge. If American literary awards don’t include British writing then more fool them. It’s just another example of America First. Surely we don’t want to encourage a Britain First mentality here.”

Since the rule was changed in 2014, there have been two US winners: George Saunders in 2017 for Lincoln in the Bardo, and Paul Beatty in 2016, for his satirical novel The Sellout. The other two winners – Richard Flanagan and Marlon James – are Australian and Jamaican, respectively.

Facebook Twitter Pinterest American author George Saunders, who won the 2017 Man Booker prize. Photograph: Will Oliver/EPA

Arguing that the rule change “isn’t even popular in America”, the letter refers to a 2017 article by Washington Post critic Ron Charles, who wrote in a piece titled “Dear Britain, Please Take Your Booker Prize Home”: “For any serious reader of fiction in this country, the Americanisation of the Booker prize is a lost opportunity to learn about great books that haven’t already been widely heralded.”

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The letter also voices the publishers’ concerns that the diversity of the prize has been hit, comparing the 2017 shortlist – which saw three US authors among the six finalists – with the 2013 shortlist, which included authors from Britain, Ireland, New Zealand, Zimbabwe, an American-Canadian and a British-American.

“In a globalised but economically unequal world, it is more important than ever that we hear voices not from the centres. The rule change has made this much less likely to happen,” the letter reads, finishing with a plea: “As concerned friends, and as publishers who worry about a homogenised literary future, we urge you to reconsider your decision”.

The Man Booker Foundation responded to the draft letter with a statement disputing the letter’s focus on US authors. “The Man Booker prize expanded in 2014 to allow writers of any nationality, regardless of geography, to enter the prize providing that they are writing in English and published in the UK. The rule was not created specifically to include American writers,” the statement reads.

Arguing that “clear trends cannot be drawn from a mere four years of data”, the foundation said there was no evidence that diversity had been affected: “The judges … are charged with finding the best novel of the year, in their opinion, written in English. The trustees believe that this mission cannot be constrained or compromised by national boundaries.”

The prize, which celebrates its 50th anniversary this year, will announce a 13-book longlist in July.