Security guards at Xcel Energy's Monticello nuclear plant reached a preliminary agreement with their employer on a three-year wage and benefit contract, ending a dispute that prompted a lockout.

The agreement, reached between the union and G4S Security early Friday, will be presented to the 25 workers for a vote later this month.

"We are confident it will pass," said Josh Haider, head of United Security Professionals Local 2, which represents the guards. "At this point, we just want to go back to work."

The locked-out guards are expected to formally return to work early Monday.

Under terms of the deal, the guards will receive a 2.5% wage increase in the first two years and a 2.25% increase in the third year.

Haider also said the guards will retain the health benefits that were the main source of contention with G4S. G4S' wanted to cut its contributions to guards' health care premiums by nearly 50%, Haider said earlier this week.

"We are keeping our medical insurance and are at least keeping up with the cost of living as far as wages," he said after the deal was reached Friday. "That's all we were holding out for in the first place."

G4S, Xcel's security contractor, last weekend locked out about 25 security lieutenants, or shift leaders, after the guards' contract expired following several months of fruitless negotiations. The guards subsequently began to picket the plant, a key source of power for the Twin Cities.

Xcel on Wednesday asked federal labor regulators to intervene after locked-out guards extended their picketing to all plant gates. Some Xcel union employees refused to cross the guards' picket lines.

London-based G4S is a global security firm that took over Xcel's nuclear security operations at the beginning of 2019.

Members of other unions joined the locked-out security workers' picket lines this week. And several politicians had planned to join the picketing on Friday morning.

Local 2 is one of two union locals of the independent United Security Professionals, which together represent about 120 guards at the Monticello plant. Local 2's contract expired in January and had been extended twice. Local 1, which represents officers at Monticello, has a contract that runs through this year.