Hunts are buying birds of prey to try and side-step the ban on fox hunting with hounds, falconry experts warn.
Campaigners say birds are being used for fox hunting
About 20 hunts have bought golden eagles and eagle owls in the belief they are acting within the law.
Using dogs to flush wild mammals for a bird of prey to hunt is an exemption set out in the Hunting Act 2004.
The Hawk Board, which represents UK falconers, said some hunts were "behaving irresponsibly", but a hunting body said bird welfare was a priority.
The Master of Fox Hounds Association, revealing the number of hunts which have bought the birds, said it believed it had interpreted the legislation correctly.
"We didn't draft this flawed bill. Our lawyers are clear this is an exemption," said its director Alistair Jackson.
"It enables hunts to use full packs of hounds."
The government ban on fox hunting allows for falconry as a sport to remain protected.
The law does not restrict the number of dogs that can be used to flush wild mammals for a bird of prey to hunt.
But Jim Chick, chairman of the Hawk Board, said there were concerns about the practice.
"This is bringing the sport into disrepute," he said.
"Many of the hunts are using people to handle the birds who have just been on a short course. You are not competent to handle a large bird of prey after a short course.
"Secondly, a fox is not a recognised quarry for a bird of prey. It is a large animal and cannot be easily subdued so there is a big ethical issue over whether they should be used.
"An eagle is possessive and once it has caught a fox it will not let go. If the hounds are then brought in they could attack the eagle and a hound could be blinded or killed."
He added it was dangerous to have an eagle with a mounted hunt, as it could lead to a horse throwing a rider.
"A small number of hunts are behaving irresponsibly," he said.
"We feel very let down. We supported them and walked the marches and raised funds for them and now this is a slap on the face."
Act in 'chaos'
The Department for Environment, Food and Rural Affairs (Defra) said there had been complaints about the use of birds.
A spokeswoman said: "In our view the falconry exemption does not allow the use of a token bird of prey to allow hunting with dogs to continue. But it is a matter for the courts to interpret the Act."
Tim Bonner, spokesman for the Countryside Alliance, which campaigned against the hunting ban, said the move showed the "chaos" of the Hunting Act.
"The Act came into force less than a year ago but it has become increasingly clear that is impossible to interpret and to enforce," he added.
A spokesman for the Council of Hunting Associations said: "The Hunting Act permits the use of hounds to flush a wild mammal to enable a bird of prey to hunt it and some people may wish to explore this possibility.
"However, the CHA strongly advises that this method should not be used unless hunts have discussed their plans with the Hawk Board and the Hunting Office."