The Broughton Bypass is to be completed five months later than scheduled, County Hall bosses have admitted.
The major project will also cost up to £3m more than predicted, following increased cash needed for the construction of a new bridge, and “other unexpected costs”.
A new completion date of August next year has been announced for the work, originally due to be finished in March, with the delay caused by “complications related to ground conditions”.
Leaders say the complications affected work on the construction of the Brooklands Bridge, over Blundel Brook, at the southern end of the scheme.
As a result of the increased cost of constructing the bridge and “other unexpected costs”, the overall original cost of the £24m scheme has increased by up to £3m.
Agreement has been given that the additional funding will come from the Preston, South Ribble and Lancashire City Deal.
Eddie Sutton, director of development and corporate services, said: “Exploratory work was carried out as part of the design and planning process, but you can only properly know what is needed once you start work on the ground.
“Once work started on site, a more comprehensive ground survey revealed that the ground conditions were more challenging than had been indicated by the initial survey.
“We then needed specialist equipment to fully investigate the situation and understand the changes that would need to be made to the design.
“We came up with a different bridge foundation design which would have enabled us to complete on time and on budget, however the Environment Agency asked us to go back to a version of the original design.
“This has meant that we have to build a bridge with significantly deeper foundations, taking longer than scheduled and increasing the overall cost.
“We’d like to thank people for their patience while this work takes place.
“I acknowledge that this delay will mean a longer period of disruption but I hope people will see that there is a long-term benefit in getting this right.”