The Countryside Alliance claims a High Court ruling on the definition of hunting in England and Wales will make it harder to prosecute huntsmen.
Judges ruled that the law banning hunting did not include "searching" for wild animals to flush them out.
They also said it was for prosecutors to prove to the criminal standard a hunter was not covered by exemptions.
Pro-hunters said it would mean fewer prosecutions but anti-hunt campaigners said that it merely clarified the law.
Hunting foxes with dogs was banned in 2005, while dogs can follow a scent or flush out a fox but not kill it.
The High Court was asked to rule on points of law relating to the Hunting Act after appeals against prosecutions.
It noted that the Hunting Act had been a controversial piece of legislation which used up a "virtually unprecedented" amount of Parliamentary time between 1997 and 2004 and added "its application and effect remain controversial".
During hearings Kerry Barker, for the Crown Prosecution Service, argued that the meaning of the word "hunts" included "hunting for or searching for".
He told the court: "If searching for a wild mammal with dogs is not illegal, then it is difficult to see how Parliament's intention of preventing cruelty and bringing an end to the sport of hunting can be met."
But the judges ruled the term "hunts" did not include "mere searching".
Mr Kerry also asked for guidance on whether it was for the prosecution to prove defendants were not covered by exemptions to the ban - or whether the burden of proof was reversed.
Exemptions granted under the act include allowing the use of two dogs to stalk and flush out a wild mammal to stop it causing "serious damage" or the use of a terrier to flush out a fox from underground to protect game birds.
The Director of Public Prosecutions had argued that it should be for the hunter to prove he was exempt. But the judges ruled that reversing the normal burden of proof would be "oppressive, disproportionate and unfair".
They said it was for the prosecution to prove "every element of the offence charged" - with some limitations - although the defendant was still under an "evidential burden" to raise "matters of defence" for the prosecution to deal with.
In its ruling, the High Court noted the "unusual nature of this legislation", adding: "If the general aim was to ban the perceived cruelty of hunting wild mammals with dogs for sport, the ban is by no means absolute."
Countryside Alliance spokesman Tim Bonner said the ruling was "very positive".
"We have won on everything essentially. This should mean the prospect of Hunting Act offences being prosecuted will be far lower in many cases.
"We would expect there to have to be overwhelming evidence (of illegal hunting) for a prosecution even to be launched."
But the League Against Cruel Sports, which brought the prosecution against Mr Wright, said: "It is really a victory for clarity in the law. Where people said there was doubt, there is now no doubt.
"We are pleased the law has now been clarified and the backlog of hunting cases will now be able to pass through the courts."
He said his lawyers had assured him the Hunting Act "as it was intended is still in place".
The DPP had been appealing against a decision to overturn the conviction of Tony Wright, a huntsman with the Exmoor Foxhounds who was the first man prosecuted for hunting foxes.
He had argued he had acted to ensure compliance with the law and had been trying to prevent damage to livestock.
Other trials - including one involving men from the Devon and Somerset Staghounds - had been put on hold while the ruling was awaited.
A spokesman for the Crown Prosecution Service said: "We will be studying the judgment very carefully to see if it should be appealed.
"We will also be monitoring any current cases carefully."