Scanners: Officials use thermal imaging to look for stowaways

What have the two countries agreed?

The UK has struck a deal with Belgium to place British immigration officials at the Brussels Eurostar train terminal. In practice, this means the British border now extends to the heart of Belgium.

Officials will be able to stop people boarding a train if they believe they have no legal reason to enter the UK.

Why has the government sought these controls?

At the height of the asylum numbers furore two years ago, a huge proportion of applicants arrived via the English Channel or the tunnel.

The government argued to its European partners that it needed to shift controls to their countries if it was going to have any prospect of effectively managing the flow.

So this isn't the first deal of its kind?

It's actually part of a growing network of international immigration measures which have seen governments accept that domestic controls must be backed by international arrangements.

Checks will be made in Brussels before travellers board the train

In the UK, the foundation of these measures was the unprecedented agreement between London and Paris that led to the French authorities closing down the Sangatte refugee camp in Calais.

Home Secretary David Blunkett argued the Red Cross-run complex was a significant "pull factor" for people seeking to enter the UK.

The Red Cross argued it was an essential humanitarian project to protect those who would otherwise be sleeping on the streets.

The second part of the deal with France came when 230 British immigration officials began working in Calais, Lille and Paris, vetting those who seek to cross to the UK. The government says these measures stopped some 9,000 people in 2003.

But what about people who try and enter clandestinely?

Another prong of the offensive has seen the UK invest heavily in hi-tech scanning systems at ports in France, Belgium and Holland to detect those who may be hiding in lorries.

The systems include x-ray machines which scan entire trailers and monitors to detect heart beats or breathing in hidden compartments.

Taken together, the government believes the controls in France have had considerable success in cutting numbers arriving?

What's their evidence?

The number of asylum seekers entering the UK fell by 41% in the past year- but it's difficult to say for certain why.

The government says its measures have reduced the pull factors.

International migration experts say movements into Europe were already dropping, helped by the collapse of the Taleban regime in Afghanistan and the fall of Saddam Hussein in Iraq.

Government critics say would-be claimants are still arriving and working clandestinely, rather than risking claiming asylum - one of the arguments at the heart of the row which led to the fall of immigration minister Beverley Hughes.

Are these the limits to international controls?

Not at all. One of the government's most controversial immigration controls has been the introduction of vetting at Prague airport in order to stop false asylum applicants from the Czech Republic.

Sangatte: Closure part of growing EU strategy

On top of this, a deal with Spain is designed to identify would-be illegal immigrants getting on planes and heading to the UK.

In 2003 the government took part in a trial exercise, Operation Ulysses, which used surveillance aircraft and naval vessels to intercept people trying to cross from Africa to Spain's Canary Islands in dinghies.

The UK has also sent forgery experts to some Asian countries to train local authorities.

One of the most controversial measures has been the introduction of some visa restrictions on countries including Pakistan, Zimbabwe and Bangladesh.

So the thrust of policy is an international one?

Asylum and unmanaged economic migration has not just been a political storm in the UK, it remains high on the agenda across the European Union which concedes it didn't forecast the mass migrations which followed the fall of communism.

Today, uncontrolled migration and people smuggling involves organised crime.

While the individuals themselves may just be seeking work, many would-be migrants pay enormous sums to organised criminal gangs for their one chance at making it into Europe to work. Others, particularly young women, are being trafficked against their will.

In other words, say ministers, migration cannot be controlled unless there is international co-operation.

But co-operation beyond bilateral deals remains patchy. The EU's members agreed in 1999 to develop a joint asylum and migration strategy, dubbed "Fortress Europe" by its critics.

Some basic measures such as identity cards and fingerprinting to prevent multiple applications are being developed.

But the full policy package is still not in place because ministers cannot agree on the most controversial aspects such as deportation arrangements.

Nevertheless, the thrust remains one of pooling domestic resources to control international access points to the European Union, be they by land, sea or air.