Under the Listing Rules (LR), any person who is entitled to exercise (or to control the exercise of) 10% or more of the votes able to be cast on all (or substantially all) matters at general meetings of the premium listed company (or of any subsidiary undertaking or parent undertaking of the listed company, or of a fellow subsidiary undertaking of the listed company's parent undertaking). Certain voting rights are disregarded for this purpose such as those exercised independently in the capacity as bare trustee or investment manager.

The full definition is set out in LR 11.1.4AR.

Under LR 11.1.4R(1), a person who is (or was, in the 12 months before the transaction or arrangement) a substantial shareholder is a related party.

For more information, see Practice note, Listing Rules: LR 11: Related party transactions: Substantial shareholder.

The AIM Rules for Companies have their own definition of substantial shareholder.