David Cameron has announced that officials in Britain and America will review the way US-UK extradition laws operate after criticism from other politicians and the press.

The Prime Minister raised the issue with President Barack Obama after lobbying from supporters of alleged computer hacker Gary McKinnon and Christopher Tappin, the retired British businessman recently extradited to the US over arms dealing accusations.

Opponents of the current extradition arrangements say it’s unfair to Britons. Are they right?

Do people in the US get more protection from the law?

Lord Baker, the retired senior judge who wrote a detailed review of the system, says no.

In both Britain and America, a judge needs to be satisfied that there is an objective reason for the suspect’s arrest before issuing a warrant.

The principle is the same in both countries: prosecuting authorities don’t need to show hard evidence that they will later rely on in court, just information that shows that there is a good reason to make the arrest.

In the US, prosecutors need to show “probable cause”, which has been defined as “a reasonable ground for belief of guilt”.

In Britain the phrase is “reasonable suspicion”. The jurist Lord Devlin said that arises when “a reasonable man, acting without passion or prejudice, would fairly have suspected the person of having committed the offence”.

Lord Baker concluded in his report that, contrary to popular belief, “there is no significant difference” between the two tests. So it’s not true to say that the American courts demand a higher burden of proof before agreeing to send their residents overseas, according to Lord Baker.

The retired judge has his critics, among them the human rights group Liberty, who said they were “completely baffled” by his findings. And the parliamentary joint committee on human rights insists there is an imbalance in the treaty arrangements.

But campaigners have not generally offered a strong critique of Lord Baker’s conclusions on “probable cause” and “reasonable suspicion”.

Doesn’t Britain extradite more people to America?

Yes. Around twice as many people have been extradited from the UK to America than vice-versa in recent years.

Between 1 January 2004 and 31 December 2011, the US made 134 requests for extradition to the UK authorities and 75 people were successfully sent to America for trial.

The UK made 57 extradition requests to the US and 40 people were successfully extradited.

To the best of our knowledge, British courts have refused to extradite seven people since 2004 but the Americans have never turned down a request from Britain.

So there’s some evidence to support Lord Baker when he says “extradition from the United Kingdom to the United States is generally more difficult to secure than vice-versa” in that a higher proportion of requests made by Britain tend to end in successful extraditions, and the US authorities are less likely to refuse a request.

Who gets extradited?

The US embassy tries to explain this apparent one-way traffic by saying that the population of the US is five times bigger.

The logic of that is presumably that in a bigger country, there are more people to carry out crimes of various kinds in various places, so it’s natural that a proportionally larger number of fugitives would leave the US and seek refuge in various other countries.

So that argument only works if we are largely talking about Americans being sent back to America having attempted to flee the jurisdiction and take refuge in the UK.

But if we are talking about the extradition of British citizens like Gary McKinnon and Christopher Tappin to stand trial over alleged crimes carried out in or against America, the “bigger population” argument fails.

What we need to know is how many of the UK residents extradited to America are Britons like Mckinnon and Tappin, or US citizens living here.

The Home Office says it can’t supply us with up-to-date figures, but this Freedom of Information answer from 2010 suggests that a much higher proportion of UK residents deported to America are British citizens than the other way round.

Of the 62 people sent from Britain to America for trial between 2004 and 2010, 28 were British nationals or had dual citizenship.

America extradited 33 people to Britain over the same period but only three were US citizens. We don’t how many were Britons who had left this country for the US and were being sent back here for trial, but in many cases that will be a likely scenario.

Of course, there are many possible explanations for the difference in numbers. It could well be that US prosecutors are more aggressive about pursuing certain crimes than their British counterparts, but that in itself is not proof that the system is biased. 

Are suspects under pressure to plead guilty in US courts?

Perhaps, but probably no more so than they would be here.

This is a notion that has cropped up in a number of articles in recent days, and was touched on by Nigel Farage, the leader of the UK Independence Party and a friend of Christopher Tappin.

Mr Farage said: “If someone pleads not guilty (in the US), they can spend years in prison awaiting trial. There are huge legal costs and they can end up getting life imprisonment. But, if they plead guilty, they can get out in two years. I imagine they will make his stay as unpleasant as possible to make him plead guilty.”

A similar argument was put forward by Gary McKinnon’s lawyers in his House of Lords, who said US prosecutors were in effect threatening their client by pointing out that defendants in the US often agree to plead guilty to lesser charges in return for a shorter sentence.

The Law Lords rejected this point, saying: “The difference between the American system and our own is not perhaps so stark as the appellant’s argument suggests.”

The judgement pointed out that a kind of plea bargaining goes on behind closed doors in the UK too, with prosecutors and defence lawyers often agreeing off the record that a defendant will plead guilty to lesser charges.

Pleading guilty gets you one-third off your sentence, and if you “turn Queen’s Evidence” – agree to help the police catch other criminals – you can get a discount of two-thirds off your sentence or even be granted immunity from prosecution.

Can you be tried in the US for something that is not a crime in Britain?

No. The law as it stands says you can only be extradited to stand trial in America for a crime that is on the statute book here and carries a sentence of at least one year.

Some commentators have claimed that this right has been abused in the case of Richard O’Dwyer, the 23-year-old accused of breaking copyright law when he created the download website TVShack.

But the judge who allowed O’Dwyer’s extradition specifically rejected this claim, saying that offence alleged by the US authorities would be a crime in Britain too.

The O’Dwyer case does illustrate one aspect of the law which campaigners say should be changed: you can be extradited to the US even if all your alleged activities took place in Britain, provided that the crime has a direct impact on the US.

By Patrick Worrall