Keeping General Augusto Pinochet's medical tests secret breaches natural justice, the High Court in London has heard.

The claim was made in the latest legal battle over whether Chile's former dictator should be extradited to face criminal charges or allowed home.

Wednesday's hearing was the start of a challenge by the Belgian Government and six human rights groups, including Amnesty International.

They say UK Home Secretary Jack Straw should have given them access to the findings of a team of medical experts who examined the 84-year-old general earlier this month.

Mr Straw says he is "minded" to allow General Pinochet to return home because the tests suggest he is unfit to face extradition.

Belgium says that as one of the states seeking General Pinochet's extradition, it should have been allowed to see the medical evidence.

The Belgian Government entered the dispute after claims that General Pinochet was behind the jailing and killing of Chileans whose relatives now live in Belgium.

Presiley Baxendale QC, for the Belgian Government, told Mr Justice Maurice Kay that Mr Straw was in danger of breaching international law and conventions by halting extradition proceedings.

Mr Straw will not make a final decision on whether to allow General Pinochet to return to Chile while the courts hear the current challenge.

He has repeatedly insisted he cannot disclose details of the medical report because they are covered by the rules of medical confidentiality.

'Irrational'

Ms Baxendale told the judge: "It's a breach of natural justice if Belgium could not make meaningful representations because they couldn't see the medical records.

"It was irrational because at one and the same time the Secretary of State was asking Belgium to make representations and refusing to disclose the details of the report."

Richard Drabble QC, appearing for the human rights groups, said that if Mr Straw did not have the power to force General Pinochet to undergo further medical tests, he could push him towards more openness.

Mr Straw could allow the extradition to go ahead and leave the general to prove whether or not he was fit to stand trial when he got to Spain if he refused to undergo more tests, said Mr Drabble.

Jonathan Sumption QC, acting for the home secretary, insisted that the laws of the country invested the authority for sanctioning extradition with the home secretary.

He said: "The home secretary is perfectly aware of the gravity of the crimes alleged against Pinochet, but his impartial judgment in these matters is a most important safeguard for the accused and the interests of justice generally.

"He has to remind himself at every stage that General Pinochet is at the moment an unconvicted accused who must be treated fairly like every other accused, and extradition procedures and his own human rights must be protected.

"The stronger the feelings people have against General Pinochet, the more important it is that this function should be conducted with detachment and compassion."

Mr Sumption said the medical examination and report commissioned by Mr Straw was sufficient for him to carry out his public duties.

While it was possible there was a factual error in the report, this was unlikely given the eminence of the team and the fact that the report had been reviewed by the Chief Medical Officer.

It was not a strong enough possibility to outweigh the duty of confidentiality owed to Pinochet, said Mr Sumption.

The case was later adjourned.

House arrest

General Pinochet will remain under house arrest on the Wentworth Estate in Surrey until the matter is resolved.

He has been fighting extradition to Spain since he was detained in London more than a year ago.

Belgium is one of three countries, along with France and Switzerland, which filed extradition applications in the wake of the Spanish request.

A plane sent from Chile to bring the general home is still in Bermuda, awaiting a final decision by the home secretary.

A Conservative MP has said he is going to lodge a formal protest to the Belgian Embassy over Belgium's "offensive interference" in the Pinochet affair.

Gerald Howarth, MP for Aldershot, said: "It is one thing for the Government of Spain to express an interest in these matters. But I find it completely offensive for the Government of Belgium to challenge the Home Secretary, Jack Straw, in our courts over his handling of a domestic British matter, namely the medical examination of the former head of state."