The pair first announced that talks were underway about National Express acquiring Stagecoach in an all-share deal valuing it at about 445 million pounds ($611 million) on Sept. 21 and the previous deadline had been set for Oct. 19.

The Takeover Panel has now given National Express until no later than 1700 GMT on Nov. 16 to either make a firm offer or walk away.

"Discussions between Stagecoach and National Express have continued, with respective management teams and advisers working constructively to progress reciprocal due diligence," Stagecoach said in its statement.

($1 = 0.7287 pounds)

(Reporting by Sarah Young; editing by James Davey)