A freighter loaded with aid which has been partially funded by British Tamils has been intercepted and ordered to anchor by the Sri Lankan navy.

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The Captain Ali, a Syrian-registered vessel, set sail from France last month but at least half of the 884 metric tons of aid on board the freighter has been provided by Tamils living in the UK. The ship's 13 crew members and 2 passengers – one of whom is British – have been detained on board by armed officers until the Sri Lankan navy decides whether or not they can proceed to a port.

The ship's organisers say their mission to bring aid to the quarter of a million refugees displaced by the recently finished civil war in Sri Lanka is purely humanitarian. But the Sri Lankan navy intercepted the ship on suspicion that it might have been carrying supplies for the now defeated Tamil Tiger separatist movement.

Navy spokesperson Captain Mahesh Karunarathe said: "We rounded up the ship and it's being brought closer to shore for checking. We know that it was coming with no local agent, no details of the cargo and the cargo was loaded by pro-LTTE (people) in the U.K. and France."

One of the members on board the boat is Kristjan Guomundsson, an Icelandic national and a former member of the Sri Lankan Monitoring Mission, an independent organisation which kept watch over a 2002 ceasefire between the Sri Lankan government and the Tamil Tigers.

Speaking to The Independent by telephone from the Captain Ali last night, Mr Guomundsson said the ship was being stopped because the correct paperwork had not been filled out. "We are currently negotiating with the Sri Lankan navy who have treated us professionally and with courtesy," he said. "This is entirely a humanitarian mission, I wouldn't be here if it was not."

The detained British national is thought to be Udhayanan Thavarajasintam, a 51-year-old charity worker. The ship's crew are primarily Egyptian and Syrian nationals.

According to organisers in the UK, the boat was intercepted by five navy vessels at 4.30pm local time approximately 18 miles off the western coast of Sri Lanka. Mr Guomundsson said the ship had been fully searched and nothing other than aid had been found.

"It may take a few days but I'm sure this will all be resolved soon," he said.

If those on board the ship are detained indefinitely it may cause a diplomatic row between Sri Lanka and Britain, which had been increasingly critical of large civilian casualties caused by Colombo's final assault on the Tamil Tigers.

Last night the Sri Lankan government announced that it would consider prosecuting doctors who helped in the rebel-held territory for "collaboration with the LTTE". It is not yet clear whether Damilvany Gnanakumar, a British woman from Essex who was working in hospitals in the rebel-held areas will be prosecuted. She is currently being held in an internment camp and her family have not heard from her since 19 May. Because Colombo banned journalist from the front line, journalists largely relied on doctors working in the rebel-held areas to describe how thousands of civilians had been killed in the fighting.

A spokesperson from the Foreign Office last night said they had not been notified whether Ms Gnanakumar had been arrested but said they were in "regular contact" with the Sri Lankan authorities over her ongoing internment.