Tax experts were unanimous in hailing Wednesday’s decision by Britain’s top court in a test case brought by Deutsche Morgan Grenfell as a “landmark” ruling which will, at the very least, cost the Treasury hundreds of millions of pounds.

Essentially, the House of Lords was being asked to decide the extent to which claims to recover tax levied unlawfully could be backdated. What the law lords confirmed, by a four-to-one majority, was that taxpayers have six years from the point at which they discover the “mistake of law” to launch their claims.