The Sri Lankan government wants Britain to include the Tamil Tigers included on a list of banned terrorist organisations.
They warn that bilateral relations would be strained if the separatist rebels are not put on the list.
The Home Office is deliberating what organisations to ban under new anti-terrorism legislation that will come into force in February.
"The government and the people of Sri Lanka will be bitterly disappointed," if the Liberation Tigers of Tamil Eelam is not included on that list, said Foreign Minister Lakshman Kadirgamar in an interview with the Observer.
"It would be an unfriendly act that would impose a considerable strain on our relations."
The rebels have been fighting since 1983 to establish an independent homeland for the Tamil minority in the north and east of the country. More than 63,000 people have been killed in the fighting. The rebels are banned in Sri Lanka, India and the US.
One of the definitions of terrorism under the new British law is the use of threat for the purpose of advancing a political or ideological cause.
"It is so abundantly clear that the LTTE is a terrorist organisation within the meaning of that expression in the UK Act," Mr Kadirgamar said.
Once in effect, the law would restrict the activities of terrorist organisations as well as affiliated or associated bodies that collect funds and conduct anti-government propaganda campaigns on its soil.
The LTTE's international secretariat is in London. The Sri Lankan government has said that funds are often collected in Britain by front organisations and funnelled into the rebel's war chest.