Foreign Secretary Margaret Beckett tomorrow launches a bid to overturn a High Court ruling in favour of islanders who claimed the Government acted illegally in removing them from their homes.

Families exiled from the Chagos islands in the Sixties and Seventies to make way for the US Indian Ocean airbase on Diego Garcia won their victory over a Government order in May last year.

The group, many of whom have settled in Crawley, have been involved in a long legal battle to return to their home islands.

Chagos Refugee Group leader Olivier Bancoult said at the time that their court victory was a "special day, a day to remember".

The High Court granted permission to take the case to the Court of Appeal.

Lawyers for the islanders had argued that although they could not live on Diego Garcia - which houses the US airbase and is the largest of the 65 Chagos islands - they should be allowed to return to the others.

Some 2,000 residents of the British territory - who mainly worked on coconut plantations - were removed when Britain leased Diego Garcia to the US to use as a military base.

In 2000, the High Court ruled that a 1971 Immigration Ordinance banning people without permits from entering or remaining in the colony was unlawful.

But in 2004, the Government changed the procedure under which the eviction was ordered, using its so-called royal prerogative to establish an Order in Council.

It is this Order in Council - in essence a Government order - which the islanders have now had declared "null and void".