Sulphurous acid is the alternative British spelling for sulfurous acid: aqueous solutions of sulfur dioxide used as preservatives. British texts on wine and dried-fruit preservation use this form for the same active species that inhibits browning enzymes and spoilage organisms. Label-declaration duties for sensitive individuals, thiamine destruction limiting use in vitamin B1-rich foods, and maximum use levels are identical under either spelling. Recognizing the pair ensures that specifications and vintage preservation records are correctly interpreted. Because it is unstable, it exists only in solution and cannot be isolated as a pure compound.