The authorities said they wanted to ensure that bills would be paid and that the passengers would return on board if they were allowed ashore.

Nedyalko Nedyalkov, an official at the Bulgarian port of Varna, said that the Taras Shevchenko, which arrived from Istanbul in Turkey, was not allowed to anchor in the port.

"We asked for bank guarantees that all expenses for the ship's stay at the port will be paid and passengers will return on board," Nedyalkov said. "Otherwise we will not let the ship in the port," he said.

Messages in a bottle

The ship is returning to its homeport of Odessa, in Ukraine, with passengers protesting that the captain is forcing them back against their will.

Health inspectors have been on board the ship

It was earlier thought that the crew was holding them hostage. The passengers threw bottles containing messages overboard and held up banners saying they were in danger, while the ship was anchored near the Turkish city of Istanbul. Some pleaded to be allowed off.

The vessel, the Taras Shevchenko, which has more than 500 Russian and Ukrainian passengers on board, had been due to call at 12 Mediterranean ports. However, after making just one stop in Greece it began heading for home.

The BBC correspondent in Istanbul says it appears that the cruise has been cancelled because of a financial dispute between the ship's owners and the tour company which chartered it.

An official from the Istanbul port authorities said health inspectors boarded the cruise liner after receiving distress signals. They reported that passengers were in good health and there was no evidence of physical harassment or injuries - although they were clearly far from happy.

The Turkish authorities said there was little they could do to intervene because the ship was in international waters.