The Isle of Man Constabulary confirmed a road traffic collision occurred shortly before 8am on Friday morning.
Few details have been released about the incident, including its exact location, how many vehicles were involved or whether anyone was injured.
In a social media post, the force said: ‘The A18 Mountain Road is closed due to an RTC at present.
TT organisers have since confirmed that Friday's schedule has been pushed back by one hour as a result of the incident.
They said: ‘Due to a road traffic collision that occurred on the mountain section this morning, the Clerk of the Course has confirmed a one-hour delay to today’s schedule.’
The Mountain Road will now remain closed for the duration of the racing programme, with roads due to reopen no later than 9.30pm.
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