The navy says that the ship was sailing under the 'pretext of a mercy mission'
The Sri Lankan navy has seized a ship sent by Tamil groups carrying supplies bound for the north of the country.
The vessel set off from the UK in April while the final battle between the Sri Lankan army and Tamil Tiger rebels was still raging.
Organisers say it was carrying food and medicine for refugees from Tamil charities across Europe.
But the Sri Lankan government said the ship was seeking to provide logistical equipment to Tamil Tiger rebels.
In May the Sri Lankan government defeated Tamil Tiger rebels fighting for a separate homeland, bringing to an end 26 years of a bitter civil war.
About 300,000 people were displaced from their homes during the final phase of fighting.
A statement from Sri Lanka's defence ministry said that the Syrian-registered vessel, Captain Ali, was seized on Thursday off the coast of Sri Lanka.
It said the ship was travelling "under the pretext of a mercy mission" and was carrying 884 tonnes of cargo which was yet to be cleared.
The expedition was organised by a Wembley-based group, Mercy Mission to Vanni, whose website says the ship is carrying food, medicines and other essentials.
The crew says their mission is to help displaced people
Half the shipment left the British port of Ipswich on 20 April, joining the other half later near Marseilles in France.
A member of the crew on board of the Captain Ali told the BBC's Tamil service that all the crew were safe and awaiting instructions to unload the cargo.
"We are waiting in the anchorage outside Colombo city. The Sri Lankan naval personnel are very professional and courteous," Kristjan Guomundsson said.
"They looked into the documents but as far as I know it is just a normal questioning. They have not taken anyone away from the ship.
He said the ship had 13 crew members.
"I hope this journey will open up a dialogue between the Sri Lankan government and the diaspora. If that succeeds it will help the displaced people," he said.
A spokesman from Act Now, a UK-based campaign group which is supporting the consignment, rejected government claims that the ship was somehow linked to the rebels.
"We fear for the integrity of the cargo and the safety of the crew," a statement released by the organisation said.
The group said the ship was seized while in international waters and that it had support from 31 British MPs as well as numerous celebrities.