Barcelona president Joan Laporta on his club's website: 'We have sent a request to the Spanish Football Federation to ask them to put our case to FIFA and the English Football Association,'
He told Barcelona-based daily El Mundo Deportivo: 'Manchester have acted in a belligerent manner in this case and have failed to respect the effort that some clubs are making in bringing on young players.
'We are going to see this case through right to the end.'
A United spokeswoman said: 'We have done everything in the correct manner and realise there is always compensation to pay and we will deal with it when it happens.'
El Mundo Deportivo quoted Pique as saying to Radio Catalunya: 'You can see that Barca don't want to negotiate
'They say I belong to them, but I'm not a slave. If they don't want to negotiate then FIFA will have to decide what United have to pay.'