Union workers representing Atlanta fire, police and city employees have filed a lawsuit against the City of Atlanta over pension contribution hikes enacted two years ago, returning focus on pension reform that largely defined Mayor Kasim Reed’s first term.

The lawsuit — filed by a handful of union workers including Stephen Borders, president of the Atlanta Professional Firefighters union — alleges an ordinance requiring employees to pay a 5 percent increase toward their benefit plans is a breach of contract and therefore unconstitutional. The plaintiffs are seeking class action status.

City officials, facing a $1.5 billion pension liability consuming about 20 percent of its annual budget, enacted the ordinance in 2011.

The lawsuit, filed in a Fulton County Superior Court Thursday, states that by requiring employees to contribute more of their annual salary, the plaintiffs will ultimately pay “over 50 percent more to purchase the same amount of retirement benefits to which they already were entitled.”

The plaintiffs are seeking to recover the additional amount they’ve paid toward their pension funds since the ordinance took effect.

City officials do not comment on pending legislation.