DENVER , Jan 9 (Reuters) - Alaska Air Group (ALK) said on Wednesday it will begin daily non-stop service in April to Denver from the Pacific Northwest, as the airline builds on the strength of its hubs and redeploys excess plane capacity from other markets. Alaska Airlines will also start daily non-stop service between Seattle and Boston. The announcement at Denver International Airport was good news for the city where UAL Corp.'s (UAL) United Airlines controls a majority of the flights and has cut back since the September hijack attacks. "From Seattle alone, Denver now represents the fourth most popular destination for travelers," said Gregg Saretsky, Alaska's senior vice president of marketing and planning. "Our new service to Denver will build on the strength of Alaska's Seattle hub, as well as (regional carrier subsidiary) Horizon's Portland (Oregon) and Boise (Idaho) hubs. COMPETITION FROM FRONTIER A spokesman for Seattle-based Alaska Air said the airline will use larger Boeing 737-900s on some current routes like Pacific Northwest to southern California, allowing it to reduce flights to those destinations and free up some planes for other destinations. Saretsky said the new routes are part of an effort by both Alaska Air and Horizon to build on the strength of their hubs in Seattle and Portland and redeploy excess capacity in existing markets where it makes sense. Aviation consultant Michael Boyd said Denver-based low-cost Frontier Airlines (FRNT) might provide heavy competition to Alaska Air because its customer service is well regarded by travelers. Alaska Air has deferred deliveries of eight 737s, four slated for 2002 and four in 2003, in order to keep costs down in the wake of the travel slump that followed the Sept. 11 attacks. Alaska has 102 aircraft in its fleet. The airline also expanded its service to Washington, D.C., since the attacks. Alaska will maintain its current fleet size by extending leases on four MD-80 jetliners originally slated to return to their owners by the end of January. Alaska and Horizon will begin service to Denver on April 28 with six daily non-stop flights, three from Seattle on Alaska, and two from Portland, Oregon, and one from Boise, Idaho, on Horizon. Alaska's new service between Boston and Seattle will begin on April 4 with one daily non-stop flight flown with a 120-seat Boeing 737-700. Also April 28, Alaska will inaugurate seasonal non-stop service between Vancouver, B.C., and Orange County, California.