HMRC explains the term Mining as below:
The most well-known consensus system is Proof of Work, which is used by Bitcoin (amongst others). Here, the right to add a new entry to the distributed ledger is only available to the first person to solve a randomly generated complex cryptographic puzzle. That person then creates the new entry and it is shared with all holders of the distributed ledger. The time and energy required to solve the puzzle is the proof of work, the right to add the entry is the primary reward. The person with that right will be entitled to any fees available for including transactions in that entry and they will be allocated with a quantity of new tokens that are released into circulation. This process is known as ‘mining’ and serves to maintain the network of a given cryptoasset.
In their Cryptoassets Manual, HMRC confirms that where the mining activity does not amount to a trade, the sterling value (at the time of receipt) of any tokens received from mining will be taxable as miscellaneous income subject to income tax, with any allowable expenses reducing the amount chargeable.
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The receipt of cryptoasset tokens as a mining reward is an acquisition of those tokens for CGT purposes. The sterling market value at the date of receipt is the CGT acquisition cost. The acquisition of tokens is not a CGT event.
When these tokens received as rewards are later disposed of, there may be a capital gain or capital loss depending on the change in value since acquisition and the application of the share matching rules, dictating which cost is offset against a disposal.