Olivier Bancoult was four years old when, holding his mother’s hand tightly, he stepped aboard a boat taking them from their island home across the Indian Ocean to Mauritius.

It was supposed to be a short visit, but as it turned out more than three decades passed before Mr Bancoult set foot again on the island of Peros Banhos in the remote Chagos archipelago.

When the family tried to sail back they fell foul of a new immigration rule, introduced by the British colonial authorities: all Chagossians who left the islands were not allowed to return. But the trick was this: islanders were not told of this rule until after they departed from the archipelago.

This was just one of the deplorable tactics used by the UK Government, as it cleared the entire Chagos archipelago between 1968 and 1973. The 2,000 islanders were mostly dumped in the slums of Mauritius, while a top-secret US military base was built on the biggest of the islands, Diego Garcia.

In a recent letter to The Times a former High Commissioner of Mauritius, David Snoxell, described the purge of the Chagos Islands as “one of the worst violations of fundamental human rights perpetrated by the UK in the 20th century”. But perhaps just as shocking is the way in which the present Labour Government — one that readily (and rightly) condemns land dispossession by African governments in Darfur or Zimbabwe — is fighting tooth and nail to deny the Chagos Islanders the right to return to their homeland.

On the face of it, it is puzzling too. When the Chagossians — led by the tenacious Mr Bancoult — won the right to return to the islands in the British courts in 2000, the Foreign Office, under Robin Cook, initially said it would abide by the court decision.

The islanders were to be allowed back to the outer islands, while still preserving Diego Garcia as a secure US military base. Mr Bancoult and his mother managed a brief, emotional visit to his birthplace. It was, he says, “a pilgrimage”.

Yet, in 2004, ministers using executive powers overturned the court decision. An order was issued banning the Chagossians’ return — and with a nasty sting in the tail it threatened anyone going to the islands illicitly with up to three years in jail.

Such scandalous, high-handed behaviour not only stinks but, as the High Court ruled last year, it is illegal. Undeterred, the Government has fought on and after two weeks of hearings the Court of Appeal is now considering its judgment.

What has proved difficult to figure out through the costly legal process — already up to £2 million of taxpayers’ money — is what the Government has to gain by it.

Certainly, evidence was heard that post-9/11 Diego Garcia assumed new importance in the global War on Terror, with the US Government insisting in a letter to the court that resettlement of the archipelago would have a “deleterious impact on our military operations”.

But the nearest of the other islands to Diego Garcia is more than 100 miles away and so hardly constitutes an immediate security threat. And anyway, why should we take the Americans’ word for the risks involved? Tellingly, no one from the Ministry of Defence had the guts to put pen to paper and construct a convincing argument for the national interest being at stake.

In fact, the real reason for this tawdry affair is nothing as important as our defence interests. It about power — and in particular the misuse of ministerial power through the outdated and undemocratic exercise of the sovereign prerogative. In 1994 an Opposition Labour MP condemned the use of the royal prerogative as “a smoke screen by ministers to obfuscate the use of power for which they are insufficiently accountable”.

Ten years later Jack Straw was a convert. It was under his watch at the Foreign Office that these antiquated powers were dusted off to deny the Chagossians their right to return home.

The invidious nature of the Government's position was plain in the Court of Appeal hearing. There was an uncomfortable moment when, on behalf of the Foreign Office, John Howell QC argued that none of those who currently live in British overseas territories has a right of abode which “cannot be removed by Her Majesty”.

That will come as a shock to all the citizens of the Falkland Islands, Gibraltar, Bermuda and any of the other 11 territories under British sovereignty. With the flourish of a ministerial pen, the busybodies in London may yet evict the lot of you!

It is also a sad commentary that the Blair Government, which introduced the Human Rights Act, one of the most far-reaching legal reforms of modern times, and came to office advocating an ethical foreign policy, is the same Government that is now behaving in a way akin to the worst authoritarian tradition of colonial days.

But Robin Cook's undertaking to the Chagossians in 2000 may yet curb the Government's ambitions to keep power at any cost. The former Foreign Secretary has hovered like Banquo’s bloody ghost over the entire proceedings — because it was his unequivocal acceptance of the islanders’ rights that has made it so hard for ministers to wriggle out of their responsibilities.

After he resigned from the front bench as Leader of the Commons over the impending Iraq conflict, Mr Cook fought the exploitation of the same overweening executive powers that were taking the country to war without a parliamentary vote. As the epitaph on Mr Cook's gravestone says: “I may not have succeeded in halting the war, but I did secure the right of Parliament to decide on war.”

If the islanders win their case for the third time in the UK courts a fitting addition might be: “Even from beyond the grave, I helped a dispossessed people to secure their right to return home.”

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