The Beaminster Tunnel is closed because of a collapsed manhole.
The tunnel on the A3066 is expected to remain closed until at least tomorrow.
A spokesperson for Dorset Council said: "We have had to close the Beaminster Tunnel on the A3066 due to a collapsed manhole.
"Advance warning signs have been put in place and the tunnel is likely to remain closed until at least tomorrow.
"We are sorry for the inconvenience that this will cause."
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here