Illegal entry means entering, or seeking to enter the UK unlawfully in breach of the immigrations laws. It is an offence to enter the United Kingdom without entry clearance or leave. Also, if a person remains in the United Kingdom after the period of leave granted has expired, for example, working or having recourse to public funds when prohibited to do so under the terms of leave. These offences are punishable by imprisonment or a fine.

People who are in the UK but who cannot establish a right to remain under the immigration rules, or otherwise, and who cannot persuade the immigration authorities to exercise discretion to allow them to stay, can be required to leave. If they do not leave, they can be arrested and detained.