http://www.hmrc.gov.uk/ni/intro/number.htm#3When and how to apply for a National Insurance number
If you don't already have a National Insurance number you must apply for one in the following circumstances. How you apply varies, as explained below.
If you need to claim benefits and/or tax credits
If you need to claim benefits and/or tax credits (or if your partner needs to claim benefits and/or tax credits for you) you will need a National Insurance number. Your application will be made as part of the benefit claim process.
How to claim benefit - Directgov website (Opens new window)
How to claim tax credits
How to claim Child Benefit
When starting work or setting up as self-employed
If you're looking for work, starting work or setting up as self-employed you will need a National Insurance number. If you have the right to work in the UK, you will need to telephone Jobcentre Plus on Tel 0845 600 0643 (lines are open 8.00 am to 6.00 pm Monday to Friday) to arrange to get one. They may require you to attend an 'Evidence of identity' interview - see section 'What happens when you apply for a National Insurance number'.
Although you need to have a National Insurance number to start work you don't need to have a plastic National Insurance number card.
If you're a non-UK citizen and want to find out whether you have the right to work in the UK, please read the guide 'Documents you will need before you can work in the UK'. The other two guides below explain how you'll pay tax and National Insurance when working.
Documents you will need before you can work in the UK
Tax and National Insurance for the employed
Tax and National Insurance for the self-employed
If you have applied for a student loan
If you're applying for a student loan and you don't have a National Insurance number, Jobcentre Plus will arrange for you to get one. They may require you to attend an 'Evidence of identity' interview - see the later section 'What happens when you apply for a National Insurance number'.