England captain Jos Buttler says the decision to bowl first was potentially a significant error as his side were thrashed by South Africa in the Cricket World Cup.

Buttler won the toss in the sweltering Mumbai heat and then had to watch the Proteas rack up an enormous first innings score of 399-7, Heinrich Klaasen the pick of the batters with 109.

England could only muster 170 all out from 22 overs in response, with bowlers Gus Atkinson (35) and Mark Wood (43) comfortably the two top scorers.

On whether his call at the toss was a mistake, Buttler said: "Yes, potentially. You always look back and reflect on these decisions. It was obviously incredibly tough out there in the heat and you could see that. 

"Everyone put in a great shift, I still believe that 340 was achievable if we got off to a good start with the bat but yeah, potentially we should've batted first in this heat.

"Those conditions were definitely tough. The humidity and the cramps, it was a real challenge to contend with.

"A lot of things didn't go to plan in our first innings. We started nicely and then Reece [Topley] picked up that injury and we were in the unknown a little bit about whether he would come back, so had to fiddle with the overs slightly.

"A couple of our guys struggled with the heat, they fought really hard and put in a lot of effort. I thought if we could restrict them to around 340-50 on this pitch, it would be a good chase for us but they got away from us in the end."

England's World Cup defence is now in tatters, with Buttler admitting his side will have to win their remaining five group games to stand any chance of making the semi-finals.

He added: "It leaves us with no room for error. We probably have to win every other game we play from here on, that's the situation we're in."

England take on Sri Lanka next Thursday, before locking horns with host nation India three days later.