Each UK FI must register on 5 May for a GIIN to ensure that it appears on the first list published by the IRS on 2 June.

The US tax authorities introduced the Foreign Account Tax Compliance Act (FATCA) to compel financial institutions to identify and report on US persons or US tax residents to help ensure that they pay the right amount of US tax, particularly where these accounts are held offshore.

The UK's Inter-Governmental Agreement with the US Internal Revenue Service compels UK institutions to report their information to HMRC. In the light of this, each one must register on 5 May for a Global Intermediary Identification Number (GIIN) to ensure that it appears on the first list published by the IRS on 2 June.

This predates 22 December, the date by which each financial institution must obtain a GIIN to avoid any US withholding in 2015.