"The road has to be fully closed to provide a safe environment in which contractors can work. We will be re-opening the road over Christmas and New Year to try to reduce disruption.
"Six months of work may seem a long time, but this is to maintain a busy route that will lead to a safer, quieter road."
The diversion will run along the A540/ A5117 to Junction 10 of the M53, then along the M53 to Junction 5 to join the A41 to the A550. Non-motorway traffic will be directed to local diversions.
* WIRRAL Council will begin work next week on a £1m road scheme to replace the roundabout at Arrowe Park with traffic lights.
Work is due to start on Monday, and the scheme is expected to be completed in April 2006.
Work will be carried out on all approach roads to the Arrowe Park /Woodchurch Road junction.
A spokeswoman for Wirral Council said: "The roads in this area will remain open to traffic at all times during the work. However, drivers are advised to make alternative journey arrangements and to use Junction 2 and Junction 4 of the M53 whenever possible."