Tax rate for lost p45?

I've just started contracting, under an umbrella company. however they have lost my p45. I really do not want to get hammered for emergency tax. how do I resolve this?

Kevin is incorrect - you retain the top copy and hand over the bottom two copies.
Without a P45, you are going to be on emergency code - thats why its so important not to lose it!
As Kevin says, fill out a P46 and then contact HMRC to ask them to send your previous pay and tax and your code number to your company. This will take a little time, but should sort itslef in a month or so.
Rward is not correct - emergency tax is NOT 22%, that is BR. Emergency tax gives you your tax free allowance on a week by week or month by month basis.
Some of the answers on here are dangerous!

Send a copy of your details on CD to the governments pensions dept and wait for them to loose it !

Failing that phone your local tax office and ask for a copy giving reasons etc. You will need your NI number.

Fill out a P46. This is the form that you need to fill out for an employer if you don't have a P45. They should have plenty of these forms for employees to fill out. get it done ASAP and it should be fine. Anyway, didn't you keep the copy of the P45. You should only hand over the top copy of it.

Emergency tax is a situation where your entire gross salary is taxed at the basic rate of 22%.
Completion of a P46, stating that this is your only or main job, will ensure that you are taxed on a week/month 1 basis. This means that you will be given the personal allowance, lower rate band etc as if this were week/month 1 in the tax year. Unfortunately, if it has been a while since you have been employed and you might otherwise be due a rebate, or pay a reduced amount of tax, then you will need to wait until your new employer receives a notice of coding.
If your old employer has kept a copy of your P45, you could ask them to complete a duplicate.
Worst case scenario is that you submit your self assessment early during April 2008, together with your last payslip, and/or P60 details for all employments, and obtain a refund if applicable.

The previous poster is wrong, that is basic rate tax. Emergency tax will give you your tax allowance of 拢5225, but on a week/month 1 basis. This means that is payday is taken in isolation, but you still get your week's/month's tax allowance, and your 10% band.
If you are subcontacting within the building industry, you may come under self-employment, and different rules apply. Look at HMRC/subcontractors.