* Exxon Mobil's involvement adds key support for project

* Partners will share financial, regulatory aspects

* BP, ConocoPhillips vow to advance rival project (Adds comments from TransCanada, Exxon Mobil executives)

By Yereth Rosen and Jeffrey Jones

ANCHORAGE/CALGARY, June 11 (Reuters) - Exxon Mobil Corp (XOM.N: Quote, Profile, Research) said on Thursday it has joined TransCanada Corp's (TRP.TO: Quote, Profile, Research) effort to build a $26 billion Alaska natural gas pipeline, giving the long-sought project key support from one of the main Arctic gas players.

Under the deal, TransCanada will retain a majority stake in the massive project to bring North Slope gas to market. But it and Exxon Mobil will share responsibility for all technical, commercial, regulatory and financial aspects, executives said.

Exxon Mobil, one of Alaska's three main producers, and TransCanada, a major North American pipeline operator, will spend $150 million on development through the process of soliciting shippers in 2010, TransCanada vice-president Tony Palmer said. That is up from a previous budget of $83 million.

TransCanada and its predecessors have envisioned the project since the 1970s. Now it and Exxon Mobil are proceeding despite low gas prices, high costs and a competing pipeline proposal from BP Plc (BP.L: Quote, Profile, Research) and ConocoPhillips (COP.N: Quote, Profile, Research).

"It is our belief that working with TransCanada and the state of Alaska ... provides the best opportunity to bring together all the necessary parties to deliver a successful project," Marty Massey, Exxon Mobil's U.S. joint interest manager, told reporters.