Cardiff fans returning from Wembley face motorway diversions
Thousands of Cardiff City fans attending one of the biggest games in the club's history face traffic chaos returning from Wembley.
Up to 40,000 City supporters will head to London on Saturday to see their side play for a place in the Premier League.
But a section of the M4 will be closed in Wiltshire on Saturday night forcing fans to take a detour on the way home.
The Highways Agency said contractors would assess traffic levels to determine when to close the motorway.
Cardiff take on Blackpool in the Championship play-off final with the winners taking their place in the Premier League next season against the likes of Manchester United and Arsenal.
Most of the tens of thousands of Cardiff fans going to Wembley are expected to travel by road.
The M4 will be closed in both directions between junctions 16 and 17 from 1900 BST on Saturday until 1000 BST on Sunday to allow a bridge to be installed over the motorway near Chippenham.
The game is likely to finish at around 1700 BST - or longer if extra time and penalties are required - which is unlikely to allow enough time for the estimated 200 coaches and thousands of cars to have passed through.
Detours will be in place advising light vehicles to use the B4042 and A429 via Malmesbury, and heavy vehicles to use the A3102, A4 and A350 via Calne and Chippenham.
Cardiff fan Rob Owen, who is driving a coach to the game, said he hoped there was still time for the Highways Agency to change its plans.
He said: "Yes, they've set up diversions for normal Saturday evening traffic, but on that evening it's going to be very heavy traffic. Most fans will be going by car and coach.
"I reckon they should delay it by a few hours and that would overcome the problem, otherwise there will be absolute chaos."
Cardiff City Supporters' Club, which alone expects to run about 70 coaches to the game, is considering taking a different route entirely from London to avoid the motorway closure.
Travel officer Vince Alm said: "They [the Highways Agency] obviously didn't know we were going to be in the play-off final that weekend.
"When they apply for a motorway closure it takes a couple of weeks to plan. It's just unfortunate.
"We will assess it in the next day or two and we may not choose to come back that route. We may choose to use the M40."
A Highways Agency spokesperson said: "The closure of the M4 to install a new overbridge at Summerlands near Chippenham has been planned for for some time.
"Before closing the motorway on Saturday evening our contractors will assess the traffic flow levels to determine when to implement the closure.
"We ask that drivers take extra care when travelling along the clearly signed diversion routes."