Clerk of the Course Gary Thompson announced the decision shortly after 3pm as the expected poor weather began to materialise.
Racing on Sunday seems a certainty, with Saturday expected to be used as an additional practice day. The RST Superbike and first Sure Sidecar races are scheduled to take place on Saturday, but a reshuffle is expected to be confirmed soon.
So far, the majority of top contenders have only completed three full laps on their Superbike machines, with Ian Hutchinson fastest overall following a lap of 128.99mph from a standing start on Wednesday on his Tyco BMW.
Ulsterman Michael Dunlop is second fastest on the time sheets on his Bennetts Suzuki, raising his speed to 127.23mph, also on Wednesday.
Thursday’s practice schedule will be carried over to Friday and is as follows:
6.20pm to 7.10pm - Superbike/ Superstock/ Supersport/ Newcomers (all classes except Lightweight)
7.10pm to 7.35pm - Supersport/ Lightweight/ Newcomers (all classes)
7.40pm to 8.40pm - Sidecars
8.40pm to 8.50pm - TT Zero (one lap)