LONDON (Reuters) - Britain expressed support on Monday for an as-yet-unpublished proposal from Ireland for new rules to let Dublin resume returning asylum seekers to the UK, which a court had halted because of Britain's plan to send illegal migrants to Rwanda.

Prime Minister Rishi Sunak aims to deport unlawful migrants to Rwanda without an opportunity to seek asylum in Britain, a centrepiece of his legislative agenda ahead of an election within the next nine months.

The measure raises particular issues for Ireland, which has the only land border with the United Kingdom, kept open and unrestricted in a common travel zone even after Britain left the EU.

The Irish High Court ruled last month that Ireland cannot return asylum seekers to the UK because the government had not outlined whether they would potentially be at risk due to the Rwanda asylum law.

Ireland's government has said it plans to introduce a legislative fix to allow it to resume sending migrants back to Britain, although it has yet to spell out publicly how it will satisfy the court's requirements.

Britain's Northern Ireland minister Chris Heaton-Harris said London was "comfortable" with the proposed legislation, having been briefed by Irish foreign minister Michael Martin on the plans at a meeting in London.

Martin had said last week the threat of deportation to Rwanda via Britain's new flagship migration programme was causing migrants to head for Ireland instead of staying in Britain. Sunak said those comments showed the plan was already working as a deterrent.

Ireland estimates that more than 80% of people who recently applied for asylum there came from Britain over the land border with Northern Ireland.

"We will obviously continue to work with Irish colleagues on preventing and addressing abuse of the Common Travel Area (between Ireland and the UK)... including in relation to return and re-admission of asylum seekers," Heaton-Harris told reporters.

"If this legislation is just resetting us back in time to where we were and what we were dealing with, then I'm comfortable with that."

Irish Justice Minister Helen McEntee said last week that the legislation - which is due to be published on Tuesday - would not apply to asylum-seekers who entered the UK from the European Union and then travelled on to Ireland. Sunak has said Britain will not accept returns from the EU via Ireland.

(Reporting by Padraic Halpin in Dublin; Editing by Peter Graff)