Red Bull Racing team boss Christian Horner says it is unrealistic to expect Max Verstappen and Pierre Gasly to challenge Mercedes and Ferrari this season.

Having switched from Renault power to a Honda engine for 2019, Horner had previously said the move could be the "missing ingredient" for Red Bull after frustrations with its former supplier.

Pre-season testing has been positive for Red Bull but, heading into the opening race of the new season, Horner says it still has some way to go to close the gap.

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"We've got an exciting new partnership with Honda starting this weekend so it's off to a positive start, but we have to keep expectations realistic," he said at the F1 season launch in Melbourne on Wednesday.

"There's a big gap to close from Mercedes and Ferrari. But we're confident that we can do that during the course of the year."

Red Bull finished 236 points behind champion Mercedes last season, the team only winning four races across the year.