Tom Wroblewski said the stress of the past three months, as he "stood up to pressure" from the Boeing Co, politicians and the union's International leadership over the recently ratified 777X proposal, had put him in hospital twice since December 27, according to a statement from the union. (http://link.reuters.com/saf45v)

Boeing's machinist union District 751 earlier this month narrowly approved a deal for building the new 777X jetliner and wings in the Seattle area, that secured thousands of jobs but will cost workers their pensions.

Boeing has built aircraft in the Seattle area for more than 90 years.

Wroblewski had earlier criticized Boeing's offer, saying it was not better than the one union members had voted down by a 2-1 margin last year.

Wroblewski has served as president and directing business representative of International Association of Machinists and Aerospace Workers District 751, which has 31,000 rank-and-file members, since March 2007.

The union said it will select a replacement to serve out the rest of Wroblewski's term, which runs into 2016.

Boeing was not immediately available for comment outside regular U.S. business hours.

(Reporting by Sruthi Ramakrishnan in Bangalore; Editing by Gopakumar Warrier)