Recasts with Sanchez's letter, adds quotes

MADRID, June 19 (Reuters) - Spanish Prime Minister Pedro Sanchez has asked NATO to exclude Spain from raising its defence spending target to 5% of gross domestic product, according to a letter sent to NATO chief Mark Rutte seen by Reuters on Thursday.

Sanchez requested a "more flexible formula" that either makes the spending target optional or excludes Spain from its application.

"Committing to a 5% target would not only be unreasonable, but also counterproductive, as it would move Spain further away from optimal spending and would hinder the EU's ongoing efforts to strengthen its security and and defence ecosystem," Sanchez wrote in the letter.

He added that the new target proposed by the United States was "incompatible with our welfare state and our world vision".

Instead, Madrid estimates it will need to spend 2.1% of GDP to meet the Spanish military's estimated investment requirements, Sanchez said.

(Reporting by Inti Landauro and Aislinn Laing; Writing by David Latona; Editing by Aislinn Laing)

((david.latona@thomsonreuters.com; +34 918 35 68 13;))