Work on a 2.5 mile (4km) stretch of road being widened to improve links to the coast is set to overrun by at least six months.
Delays to the A21 widening have been caused by the discovery of asbestos and heavy metals in the excavated earth.
The route which links south-east London, Tunbridge Wells and Hastings has been a bottleneck close to Pembury.
The road is now due to reopen in summer 2017 but may be later, Highways England said.
Campaigners worked for years to get the £69m scheme near Tunbridge Wells approved but groups including The Woodland Trust objected to the destruction of 22 acres of ancient woodland.
'Could be longer'
Andrew Broughton from Highways England said: "The site is progressing well."
He said the delay was caused when 30,000 tonnes of earth being moved was found to be contaminated and could not be reused elsewhere on the site.
Highways England said the work had been due to be finished by March 2017.
The work should now be finished by summer 2017, Mr Broughton said.
But he warned it could be longer "if we get a really wet winter".