A group comprising leading writers, poets and academics from across the world is calling for an investigation into a law that could shut the Central European University (CEU) in Budapest.

The bill is seen as part of a wider crackdown on dissent in Hungary.

Award-winning poet and translator George Szirtes, a Briton who was born in Hungary, published the open letter to Viktor Orban, Hungary's prime minister, on April 10 - the day the law was signed.

By time of publishing early on Friday, more than 600 people - including the Irish novelist Colm Toibin, Indian poet K Sachidananda and Kurdish poet, translator and painter Choman Hardi - had signed the petition.

OPINION: Why is Hungary trying to shut down a university?

The law "reduces Europe", the letter reads. "It weakens it. It takes it one step further to the edge of disintegration. It is vital to act quickly.

"We ask for a period of intensive fact-finding into the legality of the Hungarian government's law ... and its consequences for freedom of education, and for a process of mediation, bringing the parties together around the principle of European rule of law."

The new rule bars institutions based outside the European Union from awarding Hungarian diplomas without a binding agreement between national governments.

Liberal graduate school

Universities will also be required to have a campus and faculties in their home country - although registered in the US, the CEU - a liberal graduate school of social sciences - does not have a campus there.

"We are deeply concerned about the passing of the disgraceful law intended to shut the CEU in Budapest.," the signatories said. "The law, intended for this one specific purpose, is the latest step taken by [Orban] to close out democratic institutions in the country, including press, media and NGOs."

CEU was founded by the Hungarian-born American magnate George Soros.

Tens of thousands took to the streets on April 10 in Budapest to protest the bill, signed by President Janos Ader, filling Kossuth Square outside parliament.

They called on Ader, from Orban's rightwing, populist Fidesz party, to veto the legislation.

Organisers said up to 80,000 people took part in the rally, making it the largest anti-government protest in years.

The English-language CEU has 1,800 students from 100 countries and is ranked in the top 50 universities for political and international studies in the World University Rankings list.

'Foreign agents'

Critics see the move as another attack by Orban on Soros, whom he accuses of seeking to meddle in politics and undermine Europe by promoting immigration into Europe.

Orban has alleged that nongovernmental organisations supported by Soros, including Transparency International, the corruption watchdog, and Hungarian Helsinki Committee, the rights-advocate, are "foreign agents" working against Hungarian interests.

A law expected to be passed in May would force NGOs getting more than 7.2 million forints ($24,500) a year from abroad to register with authorities.

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On April 12, the EU threatened legal action in response to "recent developments" in Hungary.

Frans Timmermans, the first vice-president of the European Commission, said that the EC would also prepare a response to the Hungarian government's "Let's stop Brussels!" survey, which calls on citizens to answer questions relating to EU policies Orban decries.

As he invited Hungarians to participate in the survey, Orban wrote: "The borders must be protected, and the regulation of taxes, wages and public utility charges must also remain in our hands."