The Badger Trust is applying for a Judicial Review of the Welsh Assembly's Government's plan to carry out a cull of badgers in north Pembrokeshire.
The charity has exchanged a series of letters with WAG since September but has now decided to press ahead.
The Trust said the three month limit for a Judicial Review was about to pass.
David Williams, Chairman of the Badger Trust, said: "It is with some reluctance but nevertheless firm resolve that we must, as an organisation dedicated to the welfare and resolve of the badger enter into these proceedings at the highest level."
Mr Williams added: "The decision is unjust and goes against the scientific evidence, and the law affords us this opportunity of challenging the legality of the Welsh Assembly Government's intention to kill badgers, and we are now taking it."
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