Landlords will have to start checking passports and visas of new tenants from Monday February 1 as "right to rent" legislation comes into force.
Families taking in lodgers also have to make sure the person has the right to live in the UK.
Fines of up to £3,000 apply where tenants are not entitled to live in Britain.
The legislation, which came in under the Immigration Act 2014, is being rolled out across the country after a six month pilot in the West Midlands.
What do I need to check?
You need to check that anyone over the age of 18 has the right to be in the UK and rent a property, and that your property is the tenant's only or main home. This doesn't apply to second homes or holiday lets.
Checks apply only to new tenancies, and you have to carry out the check within 28 days of the start of the tenancy.
However, if someone has documentation which expires before the end of the tenancy, you need to make sure it's renewed.
If they have a temporary right to rent which then runs out, you need to make a follow-up check. If you find out they no longer have a right to rent you need to tell the Home Office.
What could happen if I don't?
The Home Office says you could be fined a civil penalty of up to £3,000 if an illegal immigrant is found living in your property, unless you can show that you carried out the necessary checks.
The penalty is £1,000 for a first-time offence and up to £3,000 for subsequent offences.
The penalties for lodgers are smaller: £80 for a first time penalty and up to £500 for further offences.