The Battle of France had already begun when 37 S/L Regiment began embarking. Regimental HQ and a composite battery comprising one troop from each battery embarked at Dover on 15 May 1940 and landed at Dunkirk the next day. They were followed by 307 Battery, but the rest of the regiment was halted on the road to Dover and returned to Aldershot. The Composite Battery and 307 Battery eventually joined RHQ, driving through streams of refugees, but because of the rapid advance of the Germans, they were withdrawn to take up positions protecting Le Havre (307 Bty with 24 lights) and Harfleur (Composite Bty with 13 lights), under 3 AA Bde while the main BEF was being evacuated from Dunkirk. On 9 June RHQ and Composite Bty moved again to St Malo, where 37 S/L RHQ took command of the heavy and light AA guns of 23 AA Battery, RA, and was joined by one troop of 307 Bty from the south side of the River Seine.