The judgment on Pinochet set a process in train that could take years - and rewrite Britain's laws
Jack Straw was sitting in his House of Commons office dealing with correspondence when the Law Lords stood up to give their verdict on General Augusto Pinochet. Watching the former Chilean dictator's fate being decided in the House of Lords on television, the Home Secretary may not have witnessed the gasps from the public gallery in person, but he knew exactly what the reaction would be. And he also realised immediately that he was facing one of the most important decisions of his life.

This weekend officials at the Home Office's extradition unit are sifting through representations from Pinochet's supporters and opponents - both sides equally passionate in their arguments. The civil servants will provide Mr Straw with a paper, putting the case for both sides. The Home Secretary, himself a barrister, will also be given advice on his options by James Turner, a leading QC. But at the end of the day this is a decision that only Mr Straw can make. He will not even be consulting Tony Blair. "Of course it's a big thing," one friend said. "But part of his job is making decisions about people's individual liberty, whether it's an immigration case or a warrant signing. He makes the decisions on very strict criteria. This is a legal process."

THE HOME Secretary genuinely had no idea which way the five Law Lords would jump when they rose to give their opinion on Wednesday - Pinochet's 83rd birthday. He watched the dramatic series of statements in the House of Lords with bated breath. Like everyone else, he thought that the former dictator was off the hook as the first two Law Lords, Slynn and Lloyd, rose to dismiss the appeal. Then came the dramatic turnaround as Lord Nicholls - the shy academic lawyer who held the casting vote - and Lords Steyn and Hoffman upheld it.

Mr Straw knew that this 3-2 verdict in favour of allowing Pinochet to be prosecuted would be far more difficult for him. Insiders had been speculating in recent weeks that the Home Secretary would eventually let Pinochet fly home because he would not want to be seen to be interfering in Chile's domestic affairs. The arrival in London on Friday of Jose Miguel Insulza, the Chilean Foreign Minister, shows what a diplomatic minefield the Government is entering. But, following the Law Lords' ruling, it will be far more difficult to prevent extradition. Ministers now privately admit a touch of regret that the verdict did not go the other way.

With typical diligence, Mr Straw had prepared his reaction for both outcomes well in advance. There was a statement written in case the Law Lords agreed with the original court ruling that Pinochet should not, as a former head of state, be extradited. The Home Secretary would have told MPs that the Government could not go against legal advice and the Chilean aircrew waiting near the Grovelands Priory hospital would have flown their senator home.

But there was also a strategy prepared in case the Law Lords overturned the original judgment and ruled that there were some crimes for which nobody should be immune from prosecution. Several days before the Law Lords announced their verdict, the Home Secretary had applied to Bow Street magistrates' court for an extension to the deadline by which he would have to reach a decision if this happened. He had written to the magistrate asking for more time to sift through all the representations he knew he would get. "No one had any idea which way it was going to go," a Home Office source said. "We had to prepare for both eventualities."

Labour MPs are now saying that Mr Straw's hands are tied. It would be inconceivable, they argue, for him to go against the judgment of the most senior lawyers in the land. But Downing Street stresses that no avenue has been closed off. A spokesman said that the Home Secretary does not have to think about overruling the Law Lords as he is assessing different things. While the Law Lords were examining whether Pinochet should be immune from prosecution as a former head of state, Mr Straw has to decide whether or not the former dictator is eligible for extradition. He will do so, all government departments agreed, according to the "legal process".

There are four criteria on which the Home Secretary has to base his decision. He must assess whether the alleged offences - in this case murder and torture - are extradition crimes, and whether the extradition request has been properly authenticated. He must also decide whether the offences are political (the French courts used this criteria to prevent the extradition of the former MI5 agent David Shayler earlier this month), and examine whether there are any compassionate circumstances which should allow Pinochet home.

THIS LAST category is likely to be the most controversial one. Pinochet's lawyers plan to argue that the former dictator is not in a fit mental state to undergo extradition. They are likely to use medical testimony to claim that the former dictator, while physically recovered from his back operation, is still stressed. Pinochet's supporters are also certain to highlight the fact that the Home Secretary refused to issue an order to proceed with extradition against the IRA terrorist suspect Roisin McAliskey on compassionate grounds.

If Mr Straw decides that the extradition can go ahead, there will be yet more lengthy legal wrangles. Some experts claim that Pinochet could be in Britain for two more years while the appeals and counter-appeals are batted back and forth through the courts. First the magistrate, now scheduled to see the former dictator on 11 December, will have to decide whether the offences are extraditable. If the magistrate agrees to remand him in custody to await extradition Pinochet can then apply for habeas corpus (an application for release on the grounds of unlawful detention) and judicial review. This stage would take months, if not years. Finally, if all these appeals are unsuccessful, the Home Secretary then has to assess the case again before the extradition actually goes ahead.

Baltazar Garzon, the Spanish judge who drafted the formal application for Pinochet's extradition, has a long wait before he knows whether or not he will get the former dictator in the dock. But even if the Spanish government's request falls, the case will not be closed. Pinochet would then have to fight off similar arrest warrants from the Swiss, French and the Belgian authorities.

Mr Straw will be busy for some time. But he is not the only government minister preoccupied by General Pinochet. Robin Cook, the Foreign Secretary, and Lord Irvine of Lairg, the Lord Chancellor, are now considering whether the State Immunity Act needs to be rewritten in the light of the case. The fact that the lawyers could not agree on the interpretation of the legislation has highlighted a grey area which ministers believe may need to be clarified. They believe it is astonishing that the Lord Chief Justice, Lord Bingham, could interpret the law to mean that Pinochet was immune from prosecution because he was a former head of state but the Law Lords could use the same piece of legislation to rule that he could be prosecuted because of the nature of his crimes. They are concerned that Britain will become a "haven for dictators" unless the law is made absolutely black and white. Government lawyers are now examining the 1978 Act in the light of the judgments in the Pinochet case. They are considering specifying particular crimes - such as torture or genocide - in the Act for which former heads of state would not be immune from prosecution. "The law has not been tested until now," a Foreign Office spokesman said. "Lawyers are looking extremely carefully at the judgment to see if there are implications for our legislation."

Mr Blair says that his cabinet is now beginning to "grow into government" and leave behind the attitudes of opposition. This is the first big test of that. Mr Straw almost certainly privately agrees with Peter Mandelson that it would be "gut- wrenching" if Pinochet escaped extradition. He must put his personal feelings on hold and allow the "legal process" to go ahead - although, of course, this, as the conflicting views of the Law Lords show, is all a matter of opinion.