When did the current naming convention for roads in the UK start? Eg. A1, B202, A14

Question

When did people start using A# and B# for roads?

Answer 1

Initial work on classification was begun in 1913 by the government's Roads Board to determine the quality and usage of British roads. This work was interrupted by the First World War1 and did not resume until the Ministry of Transport was formed in 1919 and given authority to classify highways, to allocate funding for road maintenance, authority for which was granted by section 17 (2) of the Ministry of Transport Act 1919.

Answer 2

The A1 was designated as the A1 in 1921 by the Ministry of Transport, although it already existed as The Great North Road.

I assume that was the start (logically A1 will have been the first).