If HMRC does not have enough information to issue a full tax code, an emergency tax code is generated until more information is received
If your personal circumstances change, there may not be enough time to get a full tax code before you need to make a payment. To bridge the gap, your umbrella employer will be told to use a temporary or emergency code. This code is indicated by a ‘W1/M1’/X suffix on your tax code (i.e. 1257 X). This stands for ‘Week 1’ or ‘Month 1’ and indicates that HMRC has asked your employer to operate your code on a non-cumulative basis.
Once they have more information, the tax code can be adjusted and any corrections made.