When did planning for a Dunkirk evacuation begin?

How far ahead did contingency planning for the evacuation begin, and was it based on a First World War plan?

It was not simply a reused First World War plan. Formal work is associated with late April 1940, but Ramsay's staff may have begun conceptual planning earlier that month.

There is no claim of documentary certainty for an earlier start date. Ramsay habitually wanted a concept and schedule for every foreseeable contingency before a crisis forced a reaction. That habit could explain how charts, routes, buoys, personnel, and shipping were organised at the speed the evacuation required. Work in early April remains a reasoned possibility rather than a proven date or a recycled Great War plan.