Tax on RSUs granted in the US

Hello, I was granted some RSUs in 2018 while working for a US company in the US. The RSUs vest in Dec 2023. I have just relocated to the UK permanently with the same company. When the RSUs vest how will tax be split between the UK and US. I have been advised that the US will claim tax on the proportion of time between grant and vest, and UK will claim tax on remaining time. Is that correct?

I have just found this document https://www.gov.uk/hmrc-internal-manuals/employment-related-securities/ersm161320 Can HMRC confirm this is correct and that "time apportionment" is the right way to think about this?

Hi Jonathan Marchini,
It is.
Relief can be claimed in respect of the USA tax though.
As the payment is from your employer, the income should be shown in the employment section if it is included in your P60.
You would then claim credit for the Tax in the foreign section under 'Employment, self-employment and other income which you paid foreign tax on'.
If it's not included in your P60, please include it on the box on the employment page for 'Tips and other payments not included on your P60'.
ERSM20193 advises that when RSUs payout at the market value on what is called "dividend equivalents" in either cash or shares, such payments will generally be taxed as earnings in the year they are received:
ERSM20193 - Employment-related securities and options: what are securities: RSUs and dividend equivalents
Thank you.

Hi,

No, as that is only up to 2015. You can see further guidance here:

HS305 Employment-related shares and securities — further guidance (2023)

Thank you.