The agency has held the people-management business-improvement scheme's standard award for more than ten years.
To attain the bronze level P&MM had to provide 26 evidence requirements, in addition to the 39 that are required for the standard level.
P&MM group human resources manager Michelle Lynch said: "We felt that we were ready to aim higher. The most significant changes that have been implemented and evident this time were in recruitment, reward and recognition with people committed to success having a sense of ownership and pride in working for P&MM".
P&MM executive director Nigel Cooper said: "This is a major achievement for the business and one for which we have worked very hard. It is a clear demonstration of our commitment to continuous improvement and we look forward to building on this further in the coming years."