The charity is contesting the reasoning behind the proposed cull
A charity is seeking a judicial review of a decision to order a cull of badgers in parts of west Wales in an attempt to eradicate bovine TB.
The Badger Trust has written to Wales' Rural Affairs Minister Elin Jones asking her to suspend the process which would see a cull in Pembrokeshire.
The trust says the plan is not backed by scientific evidence.
The assembly government said it had received a letter from solicitors and would be considering its response.
The minister said in September she would sign an order giving the assembly government the power to implement a cull, which came into force on 21 October.
The cull is being planned for an area of north Pembrokeshire, and Ms Jones said it would not take place before next April.
But the Badger Trust said the cull was at odds with "the overwhelming preponderance of scientific evidence".
A motion has been put forward against the order and a debate is being held on Wednesday in the Senedd chamber, the charity added.
Mr Williams said: "We have embarked on the process of judicial review with great reluctance because of our constructive relationship with the Welsh Assembly Government and the ministry for rural affairs generally, and we hope our scientifically-based concerns about the proposals to kill badgers will be taken seriously and that our claim will not need to be pursued.
"Our letter before action carries full references to scientific sources of information, unlike material we are aware of produced in support of a culling policy."
The trust said it had asked the assembly government in September what percentage of badgers were to be killed but had been told initial surveys had not been done and there had been no ecological surveys.
Ms Jones said when she announced the order was to be signed that more preparatory work was needed to establish exactly which areas would be suitable for the cull.