The carrier had planned to operate eight flights a week from 21-31 July, nine a week from 1-8 August and 10 a week from 9-29 August, Routes Online reports.
However, forward sales to Sri Lanka have yet to recover despite the lifting of UK Foreign Office advice against all but essential travel on June 6.
This means Sri Lankan’s Heathrow-Colombo service will now operate to its current single daily schedule, with a 21.30 departure from London.