An update from the organisers said the incident happened between the bottom of Bray Hill and Selborne Drive at Ago's Leap. No further information has been provided.
Nottingham brothers Ben and Tom Birchall were leading at the time from Peter Founds and Jevan Walmsley, with Manx crew Ryan and Callum Crowe in third.
Earlier, the second Monster Energy Supersport race was run over two laps, with Michael Dunlop holding off Peter Hickman for his 21st TT victory and a record ninth success in the class.
The blue riband Milwaukee Senior TT, scheduled for 2.15pm over four laps, was moved to a 4pm start time.
However, the race was later called off due to poor weather and has been resheduled for an 11am start on Saturday, June 11 over six laps.