WASHINGTON, May 14 (Reuters) -

Boeing's ( BA ) first Starliner mission carrying humans to

space was delayed to May 21 over issues with the spacecraft's

propulsion system, Boeing ( BA ) said in a statement on Tuesday.

Starliner's mission carrying two NASA astronauts to

space had been set for liftoff from Florida last week until a

technical issue with the rocket prompted a delay to Friday, May

17, the latest postponement for a program years behind schedule

and over $1.5 billion over budget.

A new technical issue, concerning Starliner itself, has

prompted another postponement to at least next Tuesday, Boeing ( BA )

said in statement. The issue involves a leak of helium on

Starliner's propulsion system.