The caterpillar arrived in Europe in 2005 in a consignment of box plants sent from China to Germany.
It was reported in Britain five years ago after larvae were spotted in a Surrey nursery and there have been sporadic infestations since.
But now infected plants were spotted in east London and Buckinghamshire and the RHS has documented eight cases in May in Woodford and Loughton in east London and in Stoke Poges, near Slough.
There were only four cases in the whole of 2013 and the eight sightings match the number of all sightings since 2011, a sign of establishment, says the RHS.
Box blight has afflicted buxus more severely in recent years too, because of wet weather.