A New Zealand coal mining company ignored 21 warnings that methane gas had accumulated to explosive levels before an underground explosion killed 29 workers two years ago, an investigation concluded.

The official report released Monday after 11 weeks of hearings on the disaster found broad safety problems in New Zealand workplaces and said the Pike River Coal company was exposing miners to unacceptable risks as it strove to meet financial targets.

"The company completely and utterly failed to protect its workers," New Zealand Prime Minister John Key said Monday.

The country's labor minister, Kate Wilkinson, resigned from her labor portfolio after the report's release, saying she felt it was the honorable thing to do after the tragedy occurred on her watch. She plans to retain her remaining government responsibilities.

The Royal Commission report said New Zealand has a poor workplace safety record and its regulators failed to provide adequate oversight before the explosion.

At the time of the disaster, New Zealand had just two mine inspectors who were unable to keep up with their workload, the report said. Pike River was able to obtain a permit with no scrutiny of its initial health and safety plans and little ongoing scrutiny.

Key said he agrees with the report's conclusion that there needs to be a philosophical shift in New Zealand from believing that companies are acting in the best interests of workers to a more proscriptive set of regulations that forces companies to do the right thing.

The commission's report recommended a new agency be formed to focus solely on workplace health and safety problems. It also recommended a raft of measures to strengthen mine oversight.

Key said his government would consider the recommendations and hoped to implement most of them. He would not commit on forming a new agency. Workplace safety issues are currently one of the responsibilities of the Ministry of Business, Innovation and Employment.

In the seven weeks before the explosion, the Pike River company received 21 warnings from mine workers that methane gas had built up to explosive levels below ground and another 27 warnings of dangerous levels, the report said. The warnings continued right up until the morning of the deadly explosion.

The company used unconventional methods to get rid of methane, the report said. Some workers even rigged their machines to bypass the methane sensors after the machines kept automatically shutting down — something they were designed to do when methane levels got too high.

The company made a "major error" by placing a ventilation fan underground instead of on the surface, the report found. The fan failed after the first of several explosions, effectively shutting down the entire ventilation system. The company was also using water jets to cut the coal face, a highly specialized technique than can release large amounts of methane.

The report did not definitively conclude what sparked the explosion itself, although it noted that a pump was switched on immediately before the explosion, raising the possibility it was triggered by an electrical arc.

The now-bankrupt Pike River Coal company is not defending itself against charges it committed nine labor violations related to the disaster. Former chief executive Peter Whittall has pleaded not guilty to 12 violations and his lawyers say he is being scapegoated.

An Australian contractor was fined last month for three safety violations after its methane detector was found to be faulty at the time of the explosion.