Labour has announced that it would strengthen the 2004 Hunting Act if elected and remove legal loopholes that campaigners say allow blood sports to continue under the guise of trail hunting.

Convictions have fallen to historic lows, despite claims from hunting groups that the practice is as popular as ever, following cuts that “severely compromised” the work of the National Wildlife Crime Unit (NWCU).

Environmental groups have long criticised the act, saying although many cases are supported by evidence of foxes and hares being killed by hounds, they are often not followed up.

Sue Hayman, the shadow environment secretary, said austerity measures had made convictions for wildlife crimes less likely and urged the government to make hunting laws tougher in order that they are an effective deterrent.

“The government must enhance and strengthen the Hunting Act, as Labour has pledged, closing loopholes that allow for illegal hunting of foxes and hares,” she said.

“Sweeping government cuts to police forces and the lack of resource for the National Wildlife Crime Unit are having a clear impact on investigating and convicting those responsible for wildlife crimes.”

The League Against Cruel Sports, which campaigns against hunting, has claimed “dangerously flawed reasoning” is behind cases being dropped by the police and the Crown Prosecution Service, with crucial evidence “misunderstood or ignored”.

“After 30 years in the police, I know what a good case looks like,” said Martin Sims, the league’s director of investigations, formerly a chief inspector with Sussex police and the head of the NWCU.

“When it comes to hunting, there are far too many strong cases being dropped by the police or CPS where their reasoning is weak at best and dangerously flawed at worst … Too many times, crucial evidence is being misunderstood or ignored.

“The Hunting Act needs to be strengthened so it can achieve what was intended – an end to the killing of animals by hunts. So-called ‘trail’ hunting needs to be exposed for what it is – a deliberate conspiracy to circumvent the law.”

In Hampshire last November, police dropped a case involving alleged illegal hare hunting after being told by the hunt that the animals involved were rabbits, which are covered by an exemption in the act. Activists claimed video footage showed a hare being ripped apart by hounds as a huntsman stood close by.

In another abandoned case, activists alleged there was clear video evidence of members of a hunt in Yorkshire interfering with a badger sett – a move that could stop foxes escaping underground. However, the CPS said there were problems identifying the suspects, even though they were recorded referring to each other by name, and “the evidential test for prosecution was not met”.

In the same county in January, a huntsman was recorded appearing to encourage hounds to attack foxes with horns and voice calls, but the case was dropped by the CPS, which said “it was not possible to prove the suspect was in control of the hounds in pursuit of a wild mammal”.

Facebook Twitter Pinterest Hounds are exercised by a huntsman and his assistant in Cheshire ahead of the start of the hunting season. Photograph: Christopher Furlong/Getty Images

On Boxing Day last year, when 250,000 people were said to have participated in or supported meets, Cheshire police were passed footage that campaigners claimed showed a fox being chased and then torn into pieces by hounds as hunters celebrated.

The hunt’s activities were monitored and documented by the Hunt Saboteurs Association, a non-violent direct action group that aims to disrupt illegal hunting activity.

However, the CPS ruled there was insufficient evidence, leading to the Cheshire police commissioner ordering a review into the way hunting laws are upheld following a series of unpunished fox killings and evidence that hounds were still being trained to kill.

A CPS spokesperson said: “We are committed to prosecuting wildlife crime, including unlawful hunting. We carefully consider all cases passed to us by the police and will not hesitate to charge where there is sufficient evidence and it is in the public interest to do so.”

Since the act was passed, the practice appears to have continued, with long-established hunts saying they are following a pre-laid scent. However, significant amounts of fox urine or artificial liquids would be required for each hunt, and the Countryside Alliance, which calls for anti-hunting laws to be repealed, has repeatedly refused to allow the Guardian to visit a preparation site.

In parts of the country, recorded hunting offences are on the rise, yet this is not reflected in the number of overall prosecutions, which more than halved from 127 in 2014 to 52 in 2017.

A Countryside Alliance spokesperson said: “Hunts comply with the law in line with the Hunting Act 2004, but despite this, there are numerous spurious allegations made regarding illegal hunting.

“This is largely down to a lack of understanding of the law and members of the public not realising that people following hounds across the countryside on horseback is still a perfectly legal activity.

“There have been just 24 convictions under the Hunting Act relating to registered hunts since the act was enforced in February 2005 – this is despite more than a quarter of a million days’ hunting performed by over 250 hunts. These figures do not suggest that hunts are breaking the law.”

Last year, Theresa May said she had “always been in favour of foxhunting”, but plans to give MPs a free vote on the issue were later abandoned. The government has said there are no plans to amend the act during this parliament.

Scotland has its own hunting legislation.