Simon Mann, the British former SAS officer accused of plotting a failed coup in Equatorial Guinea, is in prison in the West African nation, one of his lawyers confirmed today.

Mr Mann, 55, had lost contact with his legal team after he was snatched from a maximum security cell in Zimbabwe last week only hours after losing an appeal against extradition. But in a dramatic opening to a case before Britain’s highest court today, Philip Shepherd, QC, told the House of Lords that Mr Mann was being held in the notorious Black Beach prison in Malabo, the capital of Equatorial Guinea.

Mr Shepherd said: "This is an almost unique situation. Until early this morning, I did not know where my client was or if he was still alive."

Mr Mann, an Old Etonian and son of a former England cricket captain, is one of five defendants being pursued by the president and government of Equatorial Guinea through the British courts for damages after the plot plunged the tiny country into "mayhem". The other defendants include Eli Calil, a Lebanese businessman who is alleged to have masterminded the plot, and holding companies in the British Virgin Islands and the Bahamas.

If the appeal succeeds, it could open the door for Equatorial Guinea to gain access to bank accounts in the Channel Islands that would prove who financed the attempted coup. It would also overturn a longstanding precedent of British judges refusing to interfere in the political affairs of another state.

The question of "justiciability" is considered so important that the case will be heard by nine law lords instead of five — the first time so many have sat together since the challenge to the Hunting Act in 2005.

Opening the case, Sir Sydney Kentridge, QC, for Equatorial Guinea, told the law lords that the defendants had set out to seize control of the country's substantial oil and gas reserves for their own enrichment and in doing so meant to "kill or severely injure" the President.

Equatorial Guinea claims it lost millions after it was forced to increase security in the wake of the coup and in damage to its economy through delays to civil engineering projects and loss of foreign investment.

It alleges that between March 2003 and March 2004, Mr Mann and others hatched a plan to overthrow President Teodoro Obiang Ngeuma Mbasogo — who came to power in a violent coup in 1979 — with a force of around 70 former Special Forces soldiers armed with assault rifles, grenade launchers and mortars.

On March 7, 2004, Mr Mann and several others were captured in Zimbabwe while waiting for their plane to take off. Mr Mann claimed the arms were intended for a private security company guarding diamond mines in the Democratic Republic of Congo; however, he was convicted on firearms and immigration charges and sentenced to four years in prison. In South Africa, Sir Mark Thatcher, the son of Baroness Thatcher, later admitted financing part of the plot and was fined £265,000 and released with a suspended sentence.

The law lords will determine whether Equatorial Guinea has the right to bring a claim for damages in the UK. In particular, they will consider two complicated points of law: whether civil damages can be claimed for a conspiracy that was not actually carried out, and whether the case amounts to an exercise of Equatorial Guinea's sovereign interests.

British judges have traditionally refused to allow cases in which they consider a foreign government is attempting to enforce its laws through the UK's courts. In October 2006, the Court of Appeal ruled that Equatorial Guinea's claim did not fall within its jurisdiction because its losses were not in the form of damage to private property but were rather the result of decisions it made to protect its state and citizens.

Professor Gary Slapper, Director for the Centre of Law at the Open University, said a ruling in favour of Equatorial Guinea could risk turning the UK into a forum for settling grievances involving foreign states.

He said: "The trouble is a law court is not a good place to conduct politics, and it's a disaster for those who want to try to manage diplomatic international relations."

The hearing is scheduled to last until Thursday.

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