BP Plc will pay a record $50.6 million fine for lingering safety problems from a deadly 2005 explosion that killed 15 workers at its Texas City, Texas, refinery, the U.S. government said on Thursday.

The fine is the latest blow dealt to the London-based energy giant still reeling from the biggest marine oil spill in U.S. history, where the company faces billions of dollars worth of fines and other penalties.

The size of the fine rightly reflects BP's lack of regard for worker safety, U.S. Labor Secretary Hilda Solis said in a Thursday afternoon news conference.

BP will also spend $500 million between 2010 and 2016 to improve safety at the third-largest U.S. refinery. BP has already paid a $50 million fine to settle the criminal charges and also spent more than $2.1 billion to settle hundreds of lawsuits stemming from the explosion, that also injured 180.

Thursday's announcement comes after the Labor Department Occupational Safety and Health Administration slapped BP with 709 citations carrying a record $87.4-million in fines at the U.S. refinery in October.

As part of the agreement announced Thursday, BP denied the government's allegations that it failed to live up to the agreements on refinery safety made after the 2005 explosion.

BP's global head of refining said the company would work with OSHA to improve refinery safety.

BP has a stated goal to become a leader in process safety and we look forward to working collaboratively with OSHA to achieve an injury-free workplace in our operations, said Ian Conn, global head of refining, in a statement.

OSHA said it will continue to seek $30.7 million in fines for new safety violations found by government inspectors at the refinery.

BP is hopeful that this agreement will provide a platform to resolve the remaining citations, the company said in a statement.

The lead attorney representing victims from the BP explosion said the company should change its ways.

Hopefully someday soon management at BP will grow tired of being in the international headlines as the poster child of bad business practices and reckless abandon, said attorney Brent Coon in a statement issued by his office.

Gary Beevers, international vice president of the United Steelworkers union, which represents hourly workers at the refinery, said the agreement opened the refinery to U.S. government scrutiny.

It's like BP said here's the keys to the refinery, come in, make some coffee and look at things anytime you want, Beevers said.

BP also faces a lawsuit by the state of Texas for 40 days of excess pollution including carcinogenic benzene from the refinery in April and May.

The company was also hit with a $10 billion federal class action lawsuit earlier this month for the April-May pollution, which BP reported to state regulators.

(Additional reporting by Bruce Nichols; editing by Sofina Mirza-Reid)