UK bank is requesting completed W-9 Form - Do I use UK or US Address?

Help!! My UK bank has recently asked me to provide a completed W-9 form.

Questions: I live approximately 11-11.5 months in the UK on a civil partnership visa (since Feb 2014), so do I 1) use my UK ('living') address or my US ('mailing/billing') address on the W-9 form? 2) What are the implications of using one or the other?

Background/Context: I worked in the UK in 2013 and set up a UK bank account in early 2013. I am no longer employed, but am now living in the UK on a civil partnership visa. I'm currently collaborating with an accounting firm to finalize UK taxes from my previous 2013 work assignment (that in itself is a long story). Additionally, the 2014 US tax year be the first time I will file US taxes from the UK. That said, I am new to the tax implications of living in the UK. The first step is providing the W-9 form to the bank withing 90 days...

Can anyone please help? Thank you!

The bank's main concern on the W-9 is your US Social Security Number (Tax Identification Number). What is the address on the account? Assuming it's a UK address, IMHO that would seem to be the proper one to use.

Out of curiosity, has the bank asked you to sign any other documents?

I concur with theOAP; I would use the UK address (which I would assume is also the address you use on your US tax return).

I would use whatever address is currently the bank's address on record. The purpose of the form is to enable the bank to know you are a US person so they can report you and the balances on your UK accounts to HMRC so that HMRC can send the data to the IRS.

Thank you! Thank you! Thank you Everyone for your help! 

OAP, in answer to your question, the bank also requested me to sign a Foreign Account Tax Compliance Act Acknowledgement Slip, confirming the documents (i.e. W-9) and information (my UK address listed on the Acknowledgement Slip) are correct to the best of my knowledge.

So to your points, my UK address is the one used for the UK bank account. I will go with that one. Thank you again!

@goodtimes

Thank you very much for the information on the additional documents.

It's important to understand that the voluntary signing away of ones financial privacy rights is limited solely, via the additional documents, to the FATCA related information, and is not a blanket abdication of ALL financial privacy rights.