Spaghetti Junction disruption 'until spring 2016'
Repair work on Birmingham's Spaghetti Junction is likely to take at least twice as long as forecast and is now estimated to finish in spring 2016.
Work to replace waterproofing on the M6 Gravelly Hill Interchange began in January and was initially scheduled to finish in July.
But the Highways Agency found erosion, including on rods supporting concrete, which requires additional work.
Motorists face disruption from lanes and slip road closures.
Mathew Taylor, of the agency, said: "As you would expect of any 40-year-old structure it's going to need some maintenance.
"We have a programme of works in place to keep it in good condition and this scheme is part of that.
"It's not falling down but if we were to ignore it, it would become a significant issue."
During this work the agency said engineers had found "clusters of damage around load-bearing supports and joints", a location which made repairs "very challenging".
Mr Taylor said the maintenance work had resulted in the closure of tidal flow lanes to ensure the area was safe for highways workers and road users.
"The Aston Expressway is a unique road. There's no hard shoulder, there's no central reserve there so there just isn't the space to put up [warning] signs," he said.
A maintenance project at junction 8 of the M6, where it joins the M5, is still on schedule to finish at the end of July, the agency added.