By Dominic Casciani BBC News website community affairs reporter

The detention of asylum seekers is highly controversial

What are the facts behind the detention of asylum seekers in the UK? How does the system work and why is it so controversial?

What is immigration detention?

Under immigration law, the authorities can detain someone at any stage of their asylum claim. Immigration officials do not need judicial backing to detain someone.

The Home Office says each case is reviewed 28 days after the date of detention and then each subsequent month - although critics such as Amnesty say this may not be the whole story.

Powers also exist for the authorities to detain people prior to removal for overstaying a visa or for being in the country without any permission in the first place.

Detainees can apply for bail or judicial review at any stage, but critics say many have poor access to legal advice.

How many people can be held?

There are enough spaces to hold 2,672 people in immigration detention - about three times more than in 1997. Ministers decided to increase the use of detention in 2001 to deal with a mounting backlog of failed asylum applicants.

So how many have been held?

At the moment, the Home Office publishes a quarterly "snapshot" that shows how many people were being held on a given day.

The last figure we have is for 26 March 2005 when 1,625 asylum seekers were in detention.

Amnesty International has come up with its own figure of 25,000 for the number of asylum seekers held 2004.

It says it has estimated this from information it was given by the Home Office, and has called on ministers to reveal the true figures.

How long are they held for?

In theory detention is designed to hold someone shortly before their removal. In practice, the picture is far more mixed.

According to the snapshot on 26 March, 520 detainees had been held for two weeks or less but 45 for a year or more with no resolution of their case. More than 200 had been held for between four months and a year.

Amnesty International alleges that many detention decisions are based not on whether someone will abscond, but on whether there is a space free when the person reports to the immigration authorities as required.

Not all of those who are detained are deported from the UK. Some are released because their cases have not come to a final decision. Some of these are later found to be genuine refugees.

What about children?

In 2001, the government decided that families could be held for longer than just a few days, a decision that led to a number of high-profile campaigns against the detention of children in Dungavel, Lanarkshire.

The power to detain an individual is an essential part of protecting the integrity and effectiveness of our immigration controls Home Office spokesman

Anne Owers, the prisons watchdog, has said children should not normally be detained because of the effect on their health.

Ministers last year changed the policy again to introduce "closer and more frequent review" of family detention after one woman and her baby (she miscarried her second while in detention) were found to be genuine refugees.

So detention has gone up - but has removal too?

Ministers say public confidence in the immigration system depends on them getting removals right.

Removal of failed asylum applicants went up for two years, reaching a high of almost 18,000 in 2003/4. It's fallen back again to about 3,000 a quarter, although there are now far fewer asylum applicants to deal with than two years ago.

Last year Prime Minister Tony Blair set a target for September 2005. By then, he said, the number of monthly removals should exceed the number of applicants whose claims are unfounded. At the present rate of removals, this looks like an ambitious target.

Do other countries use detention?

Detention is used elsewhere - most controversially in Australia - and we may be seeing more of it across the EU in the years to come.

The UK has played a leading role in developing Europe-wide asylum and immigration strategies as it argues that a common solution needs to be found to managing unpredictable movements of people. The idea of international "transit camps" has been floated more than once among EU leaders, although there seems to be no likelihood of them appearing at the moment, not least because the UN's refugee agency opposes the idea.

Some countries however have tried other methods to monitor failed asylum applicants prior to removal, such as intensive reporting requirements while allowing them to live in the community.