The A421 will now be closed to all traffic from 8pm Friday, May 31 until 6am Monday, June 3, for upgrade work.
The closure from Eagle Farm Roundabout to Junction 13 of the M1, was originally set for Friday May 17 to Monday May 20.
This is to allow the Cranfield Road bridge to be demolished.
Central Bedfordshire Council have reminded drivers that the Cranfield Road bridge was closed this Monday.
The bridge will remain closed for up to 12 months whilst the existing bridge is demolished, and a replacement permanent road bridge is constructed.
A spokesman said: "Contractors will try to shorten this period, if possible."
Alternative routes can be found on www.centralbedfordshire.gov.uk/transport/a421/road-closures.aspx