* Delta orders up to 70 Bombardier regional jets
* Embraer counting on new orders to maintain output (Adds details of order, background, Embraer comment)
SAO PAULO, Dec 6 (Reuters) - Shares of Brazil's Embraer SA posted their biggest intraday loss in nearly five months on Thursday, falling as much as 7 percent after rival jet maker Bombardier Inc won a major plane order from Delta Air Lines Inc.
Delta placed a firm order for 40 Bombardier NextGen jets on Thursday, with the option to buy another 30, in a deal valued at up to $3.29 billion.
With that, Embraer lost its best bet for a big order this year, leaving the world's biggest maker of regional jets with fewer options to rebuild its shrinking order backlog and keep up the current pace of production.
"While we are disappointed in this decision, we have confidence that our aircraft provide the best overall business proposition for our customers," Embraer said in a written statement. "We are confident that we are well positioned for upcoming competitions."
Without new commercial jet orders by early 2013, Chief Executive Officer Frederico Curado said last month that Embraer would not be able to maintain stable E-Jet output next year, as its order backlog hit a six-year low on thin demand.
Curado said on a September conference call that Delta was the airline most likely to make a big regional jet order this year.
Shares of Embraer were down 5.5 percent at 12.80 reais after falling as low as 12.55 reais earlier in the session. (Reporting by Brad Haynes and Cesar Bianconi; Editing by Gerald E. McCormick and Lisa Von Ahn)