A US federal appeals court has ruled that the rights of victims in BP’s fatal Texas City explosion in March 2005 were violated by US prosecutors who reached a secret plea agreement with the UK oil group late last year before consulting victims.

The victims insist the plea deal is too lenient and they should have been consulted on the terms. “We are confident that when those objections are considered, this sweetheart plea bargain will be ultimately rejected,” said David Perry, lead attorney for the victims.

The Texas City agreement was part of a $380m settlement BP made with US authorities to resolve cases being pursued by the US government following the Texas City explosion, which killed 15 people and injured 500 in and around the facility; a major spill and corrosion in its Alaska operations; and a propane scandal. The Texas City portion of the settlement involved a $50m criminal fine.

The court decision adds to BP’s troubles in the US, where a Congressional Committee is already investigating whether the plea agreement is a strong enough deterrent, protects workers and takes into account victims’ rights.