Once a migrant reaches the UK there are several things that can happen:
If they are found at the border they can be handed over to the French authorities
Once any criminal investigation has taken place the individual is passed to Immigration Enforcement officials who assess their personal circumstances
If they are deemed to be in the UK illegally but do not claim asylum the Home Office will serve them with a removal notice
Either way they could be detained until that happens
There are 13 detention centres in the UK
Harmondsworth in Middlesex is now the biggest with a capacity of 615
Where migrants are sent depends on their gender, age and vulnerability
Some cases may be Fast Tracked depending on the circumstance
Some detention centres in the UK have drawn media attention for the "prison-like" conditions it has been reported detainees are kept in.
Britain is the only country in the EU which has no limit on how long asylum seekers can be detained.
Many are "locked up in limbo" for years, a review by cross-party MPs and peers concluded.
Complaints by former detainees include:
Being locked up for up to 14 hours a day
Denied medical treatment on occasion
Allegations of verbal, physical and sexual abuse by staff
No special facilities for the vulnerable
There are also reports of several deaths in detention centres but the Home Office will only confirm one between 2009 and 2013.
The latest Home Office figures up to the year ending March 2015 show:
People were detained in centres
Were held for less than 29 days
Were held for one to two years
Have been held over two years
Children were detained
Figures show the number of people who sought asylum in the UK in the year up to March 2015 was 25,020 - of which 10,346 were granted.
The largest number of applications came from Eritrea (3,552), Pakistan (2,421) and Syria (2,222).
Of those, 85% of the Eritrean and Syrian requests were granted compared with 22% of Pakistani nationals.