Sri Lanka sent back a ship carrying an aid cargo paid for by expatriate Tamils and Tamil Tiger supporters, after intercepting the vessel entering its waters illegally, the navy said on Tuesday.
Navy ships seized the Syrian-flagged Captain Ali on Thursday 160 km (100 miles) west of the Indian Ocean island's capital and main port, Colombo, three weeks after the rebels lost a 25-year war.
"The government has the right to accept it and its cargo or reject it. It had not followed the proper procedure so government ordered it to leave," navy spokesman Commander Mahesh Karunaratne said.
The navy uncovered no illegal items, weapons or ammunition in the cargo of 884 tonnes of relief supplies, he said.
The UK-based Mercy Mission to the Vanni, which organised the ship, said in a statement it was disappointed the "cargo of desperately needed emergency humanitarian relief has been rejected."
"We assure all of our donors and volunteers that this food and medicine will reach Tamil refugees and that their efforts and donations will not be in vain," the statement said.
It also apologised for failing to ensure the shipping documents were in order. Nearly 300,000 Tamils are being held in government-run refugee camps, which are guarded by the military and from which people are not free to leave.
The Captain Ali was bound for formerly rebel-held areas, and the group that organised its voyage had first planned for it to bring aid to civilians forcibly held by the Liberation Tigers of Tamil Eelam (LTTE) in the war zone.
But the ship arrived three weeks after the military finally wiped the LTTE out on May 18, and Mercy Mission then said the cargo was destined for Tamil refugees.
The ship aroused suspicion in Sri Lanka because of the LTTE's long history of using humanitarian organisations as fundraising fronts, several of which have been shut down under anti-terrorism laws. More than 30 countries list the LTTE as a terrorist group.
Some of Mercy Mission to Vanni's organisers were linked to the Tamil Rehabilitation Organisation, which had its funds frozen in Sri Lanka on those grounds.
The navy said none of the crew of 15, including a citizen of Iceland who was a member of the Sri Lanka Monitoring Mission that kept watch over a barely-observed 2002 ceasefire, was arrested.