Sydney's M8 Motorway delays at Euston Road and King Georges road

A major Sydney freeway has been thrown into chaos after roadworks finished late, causing both lanes of traffic to be closed.

The M8 Motorway tunnel between Euston Road and King Georges Road was closed in both directions about 5am on Tuesday after overnight roadworks finished late.

The freeway was still closed at 7.30am on Tuesday morning.

Traffic is reportedly backed up for 25km and is expected to remain heavy through the morning.

'Traffic is heavy on the alternative routes, and also on the M5. Expect delays and factor in extra travel time,' the latest alert states.

Motorists are advised to consider using the M5 East Motorway between Beverley Hills & Arncliffe or the M4-M8 Link between Haberfield and St Peters is open.

The WestConnex M8 includes a tunnel linking Kingsgrove to St Peters and a new extension which connects the M8 at St Peters to the M4 at Haberfield.