By Jeffrey Dastin

(Reuters) - United Continental Holdings Inc <UAL.N> has reached a deal for the first labor contract in its history that covers all flight attendants at the company, their union and the airline said on Friday, a breakthrough after workers' protests and years of talks.

The deal with union negotiators requires the approval of leaders of United's unit of the Association of Flight Attendants-CWA before it formally becomes a "tentative agreement." They are scheduled to meet next week.

The contract, whose terms were not disclosed, will then go to about 25,000 in-flight crew members for a final vote.

Ratification would mark a victory for new Chief Executive Officer Oscar Munoz, who has sought new contracts and harmony with workers for the No. 3 U.S. airline by passenger traffic.

A contract also would be an important step toward integrating United and Continental and reducing flight cancellations.

Since the airlines merged in 2010, their crews have continued to staff separate flights. That means when a flight is at risk of being canceled because it is short-staffed, United cannot bring in reserve employees from pre-merger Continental if it is on a pre-merger United aircraft.

A ratified contract would remove "artificial barriers holding back United" from functioning well during incidents like winter storms that ground planes, said Henry Harteveldt, founder of travel consultancy Atmosphere Research Group.

United's previous management struggled to propose terms that the two flight attendant groups, which have different work rules and cultures, found palatable.

Furloughs by the airline after the merger drew workers' ire. That began to end in 2014, when United offered voluntary buyouts that more than 2,500 employees accepted.

The union entered U.S. federal mediation with United in November. It has regularly staged protests at company meetings, underscoring the strained relations the airline has with a group that represents its face to fliers.

Munoz's appointment in September created an opening, however. In his early weeks on the job, he met with workers and said people were his priority. That changed tone has won over many at the company, to the point where Munoz has described a "new spirit" at United.

"It's been a long journey," Munoz said in a news release. "Today's agreement honors the invaluable role that our flight attendants contribute to United's success."

The company recently secured deals for pilots, gate agents and baggage handlers. Mechanics still lack a single contract for the combined airline.

(Reporting by Jeffrey Dastin in New York; Editing by Bernadette Baum and Lisa Von Ahn)