I have to confess that it is a long time since I came across this situation in practice but I do not think that the legislation is at all ambiguous, even if the publication referred to is.
SSCBA 1992, Schedule 2, Paragraph (3)(1) provides that "relief shall be available under, and in the manner provided by, the folowing provisions of [ICTA 1988] -
(a) sections 380 and 381 (set-off of trade losses against general income), but only where loss arises from activities the profits or gains of which would be brought into computation for the purposes of Class 4 computations;
(b) [repealed]
(c) section 385 (carry forward of loss against subsequent profits)
(d) sections 388 and 389 (carry-back of terminal losses)..."
Paragraph 3(4)(a) goes on to say that a loss from a relevant trade (i.e. a trade within the charge to Class 4) "...shall, as far as may be, be treated for the purpose of the charge to Class 4 contributions as reducing the person's profits or gains for that year of any relevant trade, profession or vocation"
In other words, the legislation certainly does not forbid a loss from one trade being set against a profit from another in the same year for Class 4 NIC purposes. Paragraph 3(5) also has the effect of ensuring that losses which have been used against non-trading income for tax purposes are still available to be carried forward and set against future trading income for Class 4 purposes.
John Kavanagh
UK Tax Consulting Ltd
Chartered Tax Advisers
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