Income Tax on Vested Stock Options Using FMV

Hello - I have recently exercised a portion of vested stock options in my employer, a private company. The FMV at the time of purchase was greater than the strike price, meaning I need to pay income tax on the difference. My question is how do I prove what the FMV was at the time of purchase to HMRC? Given this could conceivably rise between then and the time I am able to pay the income tax through self-assessment after the tax year, I want to ensure I am only paying tax on the difference between FMC and strike price at the time of exercise.

Hi,
 

As a general rule, where an amount is chargeable to income tax as employment income in relation to employment-related securities or options, the amount will be subject to Pay As You Earn  and National Insurance Contributions (NICs) .  This means that the income will usually be included in your P60 ERSM170000 - Employment Related Securities Manual - HMRC internal manual - GOV.UK (www.gov.uk) . Under Self Assessment the onus is on the individual to determine whether or not the acquisition of, or other transactions involving, employment-related securities leads  to a tax liability' although employers must use the best available estimate of the amount chargeable to Income Tax/NIC.  You can provide additional information in the white space box on their tax return if they choose ERSM220000 - Employment Related Securities Manual - HMRC internal manual - GOV.UK (www.gov.uk) like 2 ERSM170000 - Employment Related Securities Manual - HMRC internal manual - GOV.UK How securities - including shares and options over securities - are taxed and treated for National Insurance . Thank you.