27.—(1) The Department must—
(a)set the carbon budgets for the first three budgetary periods before the end of 2023, and
(b)set the carbon budget for the fourth and subsequent budgetary periods at least twelve years before the start of the period in question.
(2) The Department may by regulations—
(a)amend the carbon budget for any budgetary period before the start of the period (and not otherwise);
(b)amend subsection (1) so as to change the date by which a carbon budget must be set.
(3) See section 54 for restrictions on the power conferred by subsection (2)(a).