Nabarro Nathanson has renewed its exclusive contract with the Coal Authority.
The deal, which covers work in England and Wales, will last for five years - two years longer than the national firm expected.
Its previous contract with the authority expired on 31 December, after four years. Nabarros has held the contract, which complements the work of the authority's in-house lawyer, since the authority was formed.
The team is led by Michael Renger, who heads Nabarros' Sheffield office and national environmental department, together with partner John Blackwell, mines and minerals specialist Niall Logan and mining litigation specialist Michael Wright.
The work will cover three main areas: environmental issues such as rising mine water, property work, which last year involved 1,000 transactions relating to former Coal Board property, and litigation resulting from subsidence cases.
It will be managed by the firm's National Centre for Law in Industry - a cross-departmental industry group including environment, property and employment law specialists.
Renger says the reappointment emphasises the strength of Nabarros' national environmental department.