The NT code is a Nil Tax code. It means that once your tax code has been changed by HMRC you won't pay any tax, but you will pay it over to the Official Receiver.
The NT code only lasts for the tax year of your BR, so if you went BR now it would revert to normal in April 2009.
If you change jobs when you are on an NT code your tax code will revert back to normal no matter where it is in the tax year.
Not everyone gets an NT code - it depends whether HMRC have time to implement it. I went BR in March 2007 and there was no time for them to change it.
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