Fifa says it is "doing everything it can" to ensure families will be seated together at next month's Women's World Cup in France.
Fans claim they have not been allocated seats together due to a ticketing error, with some saying they have been separated from their children.
The tournament's Twitter account was inundated with fans complaining, and many have contacted BBC Sport.
Fifa said the problem will affect "only a very small number of fans".
On Tuesday, it said "less than 1% of fans" who requested tickets for the semi-finals and final will be affected.
But the issue appears much more widespread, with fans saying the problem is affecting group matches and other knock-out games too.
In a new statement, Fifa said matches that were in high demand meant "the only tickets remaining were for individual seats".
It added: "Of the 1.3 million tickets on sale for the tournament it is estimated that not having side-by-side seats will affect only a very small number of fans.
"Fifa and the Local Organizing Committee (LOC) are continuing to work towards finding the best solution for all fans attending the Women's World Cup and, in particular, are doing everything they can to ensure that families will always be seated together at each and every match."
The Lionesses have said fans who bought multiple tickets from Football Association website will be sitting next to each other.
The Free Lionesses, a fanzine run by the Football Supporters' Federation (FSF), said the issue "threatens to spoil a keenly anticipated summer of football."
"Our experience of the large numbers of fans who've reported groups and even pairs of tickets bought months in advance being located in different areas of the stadium suggests that the problem is greater than the 'less than 1% of fans requesting tickets for the semi-final and final' that Fifa has claimed," said Deborah Dilworth of the FSF.
"To find out just 18 days before the tournament kicks off that friends and families who have bought tickets face the prospect of not being seated together, and especially that parents might not be able to sit with their children, threatens to spoil a keenly-anticipated summer of football."
Fifa has advised fans to contact the LOC. But in an email response to a complaint from one fan, seen by the BBC, the organising committee said seats together had been "established by default via the server following availability."
It told the fan the "only option", if they wanted to sit with their children, was to make a request at the ticket office on match day.
According to the New York Times, fans who called a customer service phone number were told to arrive at the stadium early on match day and try to swap their tickets with nearby fans.
The issue was discovered after fans were able to print their tickets on Monday.
'Are they throwing darts at a list of seats?'
These were just some of the replies we received from fans who have been separated:
Gary Waller, Sheffield: I booked two premium seats, £30 each, for the England v Japan game as a present for my girlfriend and her daughter. These were booked on 21 December 2018, at the exact same time and under the same booking. I have received my e-tickets. These are for seats row 1, seat 1 and row 10, seat 18. How on earth have they managed this? Are they just throwing darts at a list of seats? Have requested this to be rectified but I'm not expecting much joy. Congratulations Fifa Women's World Cup and French Football Association - incompetence on the grandest scale.
Carissa Rudolf, Oregon, US: My eight-year-old daughter and I are travelling from the US to France for the semis and the finals. I bought tickets the minute they posted and now we are seated separately.
Christopher Riley, Leeds: I am taking my 12-year-old daughter to two matches in Valenciennes - one in the round of 16 and one quarter-final - and we are not sat together. We're only separated by one seat but it's still ridiculous and it will be chaos on match day when everyone is swapping seats.
Katherine, California: My spouse and I purchased tickets for the semi-finals and finals the day they went on sale. There was nothing on the Fifa website warning us that we might not be seated together. We had no idea this was even possible until we received the email with our tickets yesterday. After reading other horror stories, we are actually feeling lucky that we are 'only' three rows apart. I don't know what's more galling: that Fifa did this in the first place or, now that it has been discovered, their cavalier attitude towards fixing it. It should not be up to the fans to solve this ourselves on match day. This should have never happened.
Peter McHarry, Cumbria: Two seats, 15 seats apart. Same email reply to my complaint - oh we might be able to do something on the day - well for children.
Krystyna, England: I purchased two champions packages for the semi-finals and final the day they went on sale and have two tickets in different tiers of the stadium, and I know my mum will not want to sit on her own. Not impressed at all it, feels like a lot more than 1% and has also affected other games.
Jane Scanlan: Attending with a group of nine (all visiting from the USA). It's a 50th birthday celebration. I've no idea how many, if any, of us will be seated together or even in the vicinity of each other. Completely unacceptable. At least half the enjoyment of attending events is to share the experience with friends. Fifa are a disgrace.
Have you been affected by this? Get in contact here.