Queen Star and anti-badger cull campaigner Brian May has called for the resignation of Environment Secretary Owen Paterson over the pilot cull.
The guitarist said Mr Paterson had failed to meet the test of "honesty and transparency" expected by the public.
Statistics from the Department of Environment, Food and Rural Affairs (Defra) revealed that the six-week cull, which aimed to kill 70% of the badgers in Gloucestershire and Somerset failed to meet targets.
As a result, cull operators applied to Natural England to extend the shooting period.
The Badger Trust has issued a "pre-action protocol letter" over the proposed eight-week extension of the badger cull in Gloucestershire.
An extra three weeks was granted in Somerset, where 60% of the badger population was culled.
A "pre-action protocol letter" to Natural England and the Treasury Solicitor for the Secretary of State Secretary of State for the Department for Environment, Food and Rural Affairs has been actioned by the Trust.
It is the first stage in seeking a judicial review in the High Court, the letter states:
We consider that the decision to extend the cull in Somerset was unlawful, particularly in the light of those documents.
But given that we have only just seen them and given the relatively shorter extension there, we have advised our clients that it would not now be practicable (given the court process) to seek injunctive relief to prevent it.
But that is not the case in relation to the contemplated decision in relation to Gloucestershire not least because of the much longer extension (eight weeks) being sought (and not yet granted) and the fact that our clients have now seen the nature of the arguments being relied on to justify further culling.