US SSN equivalent in the UK?

This is sort of a tax related question as it came up as I was doing my US taxes.

Is there a British equivalent to the US SSN that tracks income earned?

When we arrived, I did not apply for the National Insurance Number but recently came into some very part time work, but a month or so I will be working full time. I am now wonder if this is necessary (necessary if it is considered a US SSN equivalent).

I assumed that the NIN was just for health care and we have been 'covered' under my husband's NIN at the surgery as I was classified as a dependent when our visa started.

Is the NIN a crucial bit of paperwork or not?

An NIN is basically the UK equivalent of a SSN.

If you are working you must have a National Insurance Number. It has nothing to do with the NHS (it's an NHS number you are thinking of which is a number assigned to you in the NHS computer system and is used to track your medical records).

The NIN is used to make sure you pay the correct taxes and that you are making National Insurance contributions each month. While you are allowed to start working without an NIN you will not be on the correct tax code and will be taxed at a higher rate until you get your NIN, and will have to request a tax refund/adjustment when you get it.

See here for how to apply for a NIN:
http://www.hmrc.gov.uk/ni/intro/number.htm