No trains will run between Swansea and Cardiff on the day 30,000 football fans are due to travel between the two cities, it has been revealed.
Train companies will offer limited alternative road transport
Swansea City said it was "very disappointed" that the engineering work will coincide with the LDV Vans Trophy final at the Millennium Stadium.
Network Rail said the work between Swansea and Bridgend on Sunday 2 April had been planned for two years.
Fans using the M4 have also been warned of roadworks near Margam.
With part of the motorway restricted to 50mph, the AA is advising supporters to leave plenty of time to get there.
Swansea City director David Morgan said the club, South Wales Police and Swansea and Cardiff councils had all asked Network Rail to reconsider its decision.
"I'm sure that if the Welsh rugby team had been playing in Cardiff we would not have the same problem getting the trains running," he said.
"There was work due to start at Chepstow recently and the Welsh Rugby Union wanted it to be postponed which it was.
"It seems to be one rule for one and another rule for others - we are very disappointed at Network Rail's attitude."
Swansea season ticket holders have already snapped up more than 10,000 tickets for the game, with the club expecting to sell up to 20,000 more when they go on general sale on Thursday.
Fan Glen Donnachie said he and a group of friends had planned to go by train.
"We've had to hire a minibus to get there now - and even then we've had to go to a company outside of Swansea to make sure they don't have any bother when they take us into Cardiff."
A spokeswoman for AA Roadwatch said roadworks on the M4 between junctions 39 and 40 near Port Talbot would affect the journey east.
The final kicks off at the Millennium Stadium at 1330 BST
"Arrangements to car share with fellow fans would be advisable to limit the effect of a significant number of extra vehicles using the road network," she added.
Network Rail said the engineering works was part of a £400m project to re-signal south Wales and the line would be closed in both directions between Bridgend and Swansea all day.
It said "extremely limited" alternative road transport would be available for people with tickets.
Western route director Robbie Burns said: "For the last few years we have promised the FA that we will have no significant work on any route to Cardiff on FA Cup final day itself.
"But on any other weekend we cannot always offer that. There are a large number of events held in Cardiff that we simply cannot keep all those times work free.
"This work has been planned for two years and is necessary to maintain the safe running of the railway in south Wales."