Occupancy in many London hotels remains down by as much as 25%, according to figures from Visit London.
London hotels were initially busy, said the tourism body, but occupancy dropped off as visitors due to leave found alternative means of transport, and they were not replaced by inbound travellers.
Visit London estimated the cost to the tourism industry of the ash crisis to be around £100m, in terms of "opportunity loss" in visitor spending.
At this time of year, around 30,000 overseas visitors usually arrive by air into London every day, spending an estimated £17m a day in the city, said Visit London. Flights in and out of the city were suspended for six days.
The Mayor of London, Boris Johnson, said: "Coming out of the economic downturn, the last thing the capital's economy needed was all air traffic in and out of the city grounded for a week."
Sally Chatterjee, Visit London's CEO, said: "Although airspace has now reopened, it will still take some time for tourism businesses in the capital to see an uplift in trade from overseas visitors."
www.visitlondon.com