how long can you store share losses

If I made a loss on shares in 2021, how quickly do you get to report the losses to the HMRC, and also how long do you get to offset the losses against any gains

Hi Colin,
There are two ways in which capital gains losses can be declared to HMRC:
(a)  by submitting/amending your Self Assessment tax return or
(b) by writing to:
H.M. Revenue and Customs Self Assessment.
BX9 1AS.
You can claim losses for up to four previous tax years.
HS227 Losses (2023) provides more detailed information.
Relief for trading losses (Self Assessment helpsheet HS227)
Thank you.