By Bernie Woodall

DETROIT (Reuters) -- Leaders of the United Auto Workers union ratified a four-year labor agreement with General Motors Co on Friday, two weeks after most rank-and-file GM workers voted in favor of the new contract.

Ratification, announced by the union, was delayed two weeks because skilled trades workers, who are fewer than general production workers at GM's U.S. auto plants, had voted it down.

UAW leaders quizzed skilled trades workers on the reasons for their rejection and then went back to the negotiating table with GM seeking changes. Skilled trades workers in general maintain machines at auto plants, and include electricians, pipefitters, tool makers and millwrights.

The new contract goes into effect on Monday. It calls for raises for all workers and the end of the two-tiered pay system, although it will take a newly hired worker eight years to reach top pay rather than the three years it used to take before 2007.

Workers hired after 2007 have made less than those hired before that year.

The average labor costs, of which pay is nearly half, for GM workers will be $60 per hour by 2019, up from $55 an hour now, according to a new study by labor analysts released on Friday.

In a statement, GM said ratification of the contract was "good for employees and the business."

The UAW said its leadership ratified the deal after GM and union negotiators worked through objections of the skilled trades workers, which included "core trades classifications and seniority rights."

GM and the two other Detroit automakers, Ford Motor Co and Fiat Chrysler Automobiles, have worked for years to lessen the number of classifications of skilled trades workers.

The UAW defended the ability of a minority of its members to hold up ratification even if the majority has voted for a proposed contract.

"Since its inception, the UAW has put in place a process to ensure that minority groups have a voice," the union said.

Meanwhile, Ford's new four-year contract was still being voted on by UAW members on Friday, with the prospects for passage bleak.

On Wednesday, UAW leaders said that with three-fourths of the vote counted, 52 percent of workers were against the new pact. That was before a big auto plant in Chicago voted 68 percent against the contract.

The UAW may go back to the negotiating table if the Ford contract fails, and it could take workers out on strike.

(Reporting by Bernie Woodall; Editing by Tom Brown)