Recep Tayyip Erdogan, the Turkish prime minister, rejected demands from European leaders for it to open its borders to Greek-Cypriot shipping by the end of the year.

In one of his strongest statements to date, Mr Erdogan warned that Turkey would not move until the EU ends a trade embargo on the "republic of northern Cyprus", which is recognised by no one but the Turks since their 1974 invasion.

Speaking to the Chamber of Commerce in Istanbul, Mr Erdogan said: "Don't expect anything ... not on the subject of the ports and airports."

Brussels has warned Ankara that it must open its ports and airports to all EU countries - including Greek Cyprus - by the end of the year or risk a disruption of its membership negotiations.

Mr Erdogan made clear he was prepared to pay this price when he said: "It's astonishing that the negotiations could stop. Look, I'm being very clear: if they stop, they stop ... we will never take a step backwards on the ports or the airports without a lifting of the isolation [of northern Cyprus]."

The tough stance adopted by Mr Erdogan showed how Turkey's EU membership talks are heading for a crisis in the autumn when the European Commission delivers its annual progress report. Olli Rehn, the European enlargement commissioner, has warned of a "train crash" unless Turkey opens up its ports and speeds up reforms on human rights and free speech.

A series of European leaders warned Turkey that it must act unilaterally and not attempt to link the opening of ports to the lifting of the trade embargo on northern Cyprus. Jacques Chirac, the French president, said talks may have to be suspended unless Turkey acts.

"It is obvious to me Turkey must respect the obligations it has entered into to allow goods coming from Cyprus access to its ports," Mr Chirac said. "If it didn't, it [Turkey] would be putting in doubt itself its capacity to pursue enlargement."

The spat between Ankara and the EU capped one of the worst weeks in EU-Turkish relations since membership talks opened last October. Cyprus came close to derailing the opening of the detailed stage of the talks on Monday when it demanded that Turkey recognise its half of the island. A crisis was averted after EU foreign ministers agreed to remind Ankara that it must recognise Nicosia during the membership negotiations, which could last up to 15 years.

Unease at the prospect of Turkish membership was highlighted at the EU summit when Austria tabled proposals to put a brake on the process. Under the Austrian proposal the current requirement to consider the EU's "absorption capacity" every time a new member is admitted would become a strict criterion.

This initiative, aimed directly at Turkey which could account for 20% of the EU's population, was watered down. But European leaders will have a full debate on the EU's "capacity to absorb new members" at their next summit in December.

With the EU keen to admit less contentious new members - notably Croatia - European leaders set a timeframe to draw up a rule book for the union after the rejection of the constitution. The current rules will have to change if the EU wants to increase to 28 members - the number that will be reached if Croatia joins.

Angela Merkel, the German chancellor, will outline a detailed set of proposals in a year's time when she holds the EU's rotating presidency. A deadline for reaching a decision has been set for the second half of 2008, by which time France will hold the presidency.

By Ian Traynor and Nicholas Watt

/www.guardian.co.uk/