Six Polish swimmers have had to return from Japan even before the start of the Olympics after the Polish Swimming Federation (PZP) sent too many athletes because of an administrative error.

The swimmers returned to Poland on Sunday, threatening legal action and calling for the head of the PZP to resign over the incident.

The swimming team sent to Japan numbered 23 people.

"I would like to express my great regret, sadness and bitterness at the situation," PZP director Pawel Slominski said in a statement.

Slominski admitted the mistake but said it had been motivated by a desire to "allow as many athletes and coaches as possible to take part".

One of the six, Alicja Tchorz, wrote a post on Facebook attacking the "incompetence" of the PZP, saying they had "misunderstood the rules".

"Imagine that you sacrifice five years of your life and... your sacrifice results in a total flop," said Tchorz, who took part in the 2012 and 2016 Olympics.

Another of the six, Mateusz Chowaniec, said on Instagram: "This is an absurd situation that should never have happened."

Poland's culture and sports ministry urged the PZP president "to immediately submit explanations" over the incident.