Red Bull boss Christian Horner puts Sebastian Vettel's error-strewn recent past down to the intense expectations that come with racing for Ferrari.

Vettel has made a string of high-profile errors over the past year, triggered by his infamous crash out of the lead of the German Grand Prix last July. Those mistakes ultimately cost him the shot of beating Lewis Hamilton to a fifth world championship.

Those have continued in 2019, with two errors while under pressure from Hamilton -- he spun while fighting the Mercedes driver in Bahrain and then ran wide while defending the lead at the Canadian Grand Prix two weeks ago. The latter led to the controversial penalty for rejoining the circuit in an unsafe manner which ultimately cost him his first win since the Belgian Grand Prix last year.

Vettel's erratic form is a far cry from the dominant performances he turned in for Red Bull at the start of the decade. Horner thinks it all comes down to the environment he finds himself in now.

"With us, Sebastian was like a metronome when he was in the lead of a grand prix," Horner told Press Association Sport ahead of the French Grand Prix, where Vettel could only qualify seventh after a below-par qualifying display. "He was always so strong in that position.

"He obviously looks a lot more under pressure these days as he is carrying the hopes of Ferrari in what looks like a pressure-cooker environment."

Vettel is in the penultimate year of his current contract and rumours that he is considering retirement from F1 have refused to go away this year. That speculation only intensified after the controversy in Montreal, where Vettel said F1 as it is now is not the sport he fell in love with as a kid.

Horner refused to comment on whether he thinks Vettel will walk away, although he suggested Ferrari needs to ease the pressure on the German driver if it wants him to stay around beyond 2019.

"Sebastian is very much his own man. He is a fan of the history of the sport, and like anybody, when you enjoy what you are doing, you do it that much better.

"Certainly the years that we had at Red Bull were always a lot of fun. Even when we were competing hard against formidable rivals there was always a fun factor to it. To get the best out of people, you have got to deliver the right environment for them -- that is so important."