The High Court is ruling on a legal challenge over government backing for the culling of thousands of seagulls.
The Royal Society for the Protection of Birds (RSPB) is concerned that the cull is setting a dangerous precedent for bird conservation in the UK.
An RSPB legal team is asking a judge to quash the Environment Secretary's decision to sanction the cull in the Ribble Estuary on the Lancashire coast, at the request of aerospace firm BAE Systems.
BAE said a reduction in the population of lesser black-backed gulls and herring gulls was required because of fears over birds being sucked into the engines of jets taking off at the airfield at its Warton site.
Consent was given for the killing of 552 pairs of lesser black-backed gulls and for further operations to maintain the population at the reduced level for 10 years, provided the overall population was not reduced to lower than 3,348 pairs.
There was also consent for further measures to be taken to maintain the herring gull population at the reduced level that followed an earlier cull.
David Forsdick QC, for the RSPB, said at a hearing in London: "As far as we are aware this has never happened before in the UK and that is why the RSPB is so concerned to have the decision set aside."
Asking Mr Justice Mitting to order a rethink, he said the culls threatened to undermine the conservation purposes of European directives for birds and habitats.
The populations of lesser black-backed gulls were in "substantial decline" across the UK and in need of protection.
He suggested that, if the judge was not convinced the law was in the RSPB's favour, a reference should be made to the European courts for a final decision.
He said the Ribble Estuary was a site of special scientific interest (SSSI) covering 9226.3 hectares, or about 35 square miles, supporting internationally important numbers of waterbirds including the gulls, redshank and common tern
A special protection area (SPA), with a designated baseline for bird numbers, had been set up under the Birds Directive to encourage survival and breeding of designated species.
It was the RSPB's concern that the culls allowed reductions in bird populations "significantly below the baseline" despite the SPA purpose being to maintain it.
The Department for Environment, Food and Rural Affairs (Defra) argues the cull is both lawful and necessary because the gulls pose a threat to air safety and "human life is more important than wildlife".
BAE said in a statement the gulls "present a risk of bird strike to aircraft operating from Warton airfield" and the aim of the culls was to reduce the risk.