The people of Chagos were removed from their home islands in the late 60s and early 70s by the government of the UK when it leased the island of Diego Garcia to the US (we use it as a military base). Since then, the displaced Chagossians have been fighting for the right to return to the islands
They won the right of return in a court case in 2000, but following several years of inactivity, the government of the UK used royal prerogative to overturn the court decision, arguing that "it would not be right for the Chagossians to be allowed home because of security concerns." (That's an al Jazeera quote, not a quote from any British official.)
That use of royal prerogative has now been deemed unlawful.
Lord Justice Sedley, giving the lead ruling, said the government's use of the Order in Council under the Royal Prerogative - powers that allow action without reference to Parliament - was an unlawful way of preventing the islanders from returning.
Lord Justice Waller said the decision had been taken by a government minister "acting without any constraint".
The UK Chagos Support Association welcomed the court's decision and also urged the government not to appeal again.
Chairman Robert Bain said: "The government knows the Chagossians have no independent means to resettle the islands.
"To accept the islanders' right to return but do nothing about it - as it did between 2000 and 2004 - would be meaningless and immoral."
The government has one final avenue of appeal open to it -- the House of Lords. They have a month to decide whether or not to file such an appeal.