With 33,000 supporters heading to London for the 12.15pm kick-off, South West Trains says it may not be able to get everyone to the game on time.
Engineering work outside London means there are fewer trains into the capital from Portsmouth, as revealed in The News earlier this month.
South West Trains had been confident it could cope with the extra passengers by adding more carriages to its services.
But today it urged fans to buy their tickets now and make sure they set off early on Saturday April 5 to avoid delays.
The warning comes after British Transport Police said it expected 'extremely high' numbers of fans to go to Wembley by train.
Bill Marshall, station manager at Fratton, said: 'The early kick-off means we only have about three hours to get everyone into London for the start of the match.
'We will try to get as many people on the trains as possible but we also have to be aware of safety. There will be some people feeling like sardines on the day though.
'Our advice is to buy your tickets early and get to the stations as early as possible.
'Quite a lot of fans are travelling up the night before.'
Supporters are advised to get to London by at least 11am so they have enough time to get across the capital on the underground.
But with only two trains running every hour, instead of the normal three, this means there will only be a maximum of eight services available.
Journey times are also expected to be 30 to 40 minutes longer because of diversions.
South West Trains is laying on extra carriages, but it will still be only able to take up to 1,620 seated passengers every hour, meaning many fans will have to stand up for the entire journey.
So far 110 coaches have been booked which will take up to 5,280 supporters.
But this will leave thousands more struggling to get up to Wembley on the trains.
Driving to Wembley is also out as there is little parking at the stadium.
Barry Dewing, spokesman for Portsmouth Independent Supporters' Club, said: 'The early kick-off time seems to be the main problem. It's going to make it difficult for a lot of people but by hook or by crook they'll get there.'