The three are being flown home immediately from the tour of England, but it is understood that the series is not in any jeopardy at this stage.
Overnight, a video had emerged showing Mendis and Dickwella out in a public place. Gunathilaka was not seen in the video, but the three are believed to have stepped out together. The touring players had been categorically told that they were not allowed to go towards Durham's City Centre, but that is exactly where the trio have admitted to have gone, according to SLC officials. In the video, shot from a vehicle, Mendis and Dickwella are seen looking around the square. Although both players have their masks handy, neither is wearing them.
SLC's medical staff are understood to only have found out about the bubble-breach via social media, but had made sure to isolate the three players in their rooms when they were made aware of the excursion. The players had supposedly returned to their rooms around midnight, and are not believed to have jeopardised the biosecure bubble for others in the touring contingent.
"The players have been suspended pending a full inquiry, but have confessed to having gone out," SLC vice-president Mohan de Silva said. "They are being recalled from the tour with immediate effect."
Early indications from the SLC are that the board is viewing the breach as a serious offence and will likely impose severe penalties. Gunathilaka, in particular, has a history of poor behaviour, having been suspended for an indifferent attitude towards training in 2017 and violating curfew in the middle of a Test in 2018.
This breach of Covid-19-related health protocols, and the consequent suspension, will substantially weaken a Sri Lanka side that had already been struggling on this tour. All three players were likely to be in Sri Lanka's first-choice XI for the first ODI on Tuesday; with Avishka Fernando ruled out with a quadriceps injury, Sri Lanka have a very limited batting group available to pick from.
The team was already facing criticism for their woeful performance in the T20Is, where they were swept 3-0. None of the three had been good in the three matches. Mendis scored 54 across three innings, Gunathilaka made a total of 26 in three, and Dickwella hit 14 runs in his two outings to go with a barren run behind the stumps. They will now likely be subject to substantial criticism over their off-field behaviour.
The first of three ODIs against England is scheduled for Tuesday.
The story was updated at 10.05am GMT on June 28 following the news that the three players had been suspended.
Andrew Fidel Fernando is ESPNcricinfo's Sri Lanka correspondent. @afidelf