Captain Jos Buttler warned his England side they have "areas to improve" ahead of their one-day World Cup defence after failing to seal a series clean sweep in Bangladesh.

England lost the third and final one-day international against Bangladesh in Chittagong on Monday by 50 runs, although the tourists had already secured the series.

With an eye on the World Cup later this year in India -- where conditions will be similar to Bangladesh -- Buttler changed his England side for the final match.

Sam Curran was promoted to number five in the batting order, scoring 23 off 49 balls, and the 18-year-old leg-spinner Rehan Ahmed made his ODI debut.

The game was England's last scheduled 50-over match until September 8.

"I think we played some really good cricket throughout the series and I've spoken a lot about these being great conditions for us to challenge ourselves in," Buttler said.

"These are probably the conditions that we would find the hardest as a team.

"Now there's plenty to learn -- things that we've done well and areas that we can also improve," the captain added.

England were set 247 to win in Chittagong but folded for 196 and Buttler said that "we needed someone to go on and really take ownership of that chase".

But he had no regrets about experimenting with his line-up.

"We changed a few things today and gave opportunity to people in different ways, but I thought the intensity was there," he said.

England have some difficult decisions on the make-up of their World Cup squad.

Senior players Joe Root and Jonny Bairstow are both likely to return, while Test skipper Ben Stokes has hinted that he could come out of ODI retirement for England's title defence.

Buttler can see a possible repeat of England's build-up to last year's Twenty20 World Cup in Australia, where they emerged as champions.

"I think the schedule is hugely challenging to always get your best XI on the field," Buttler said.

"Looking back to the T20 World Cup, we probably went into that World Cup having never played our perceived best XI.

"But then to get into the tournament and go on to win it, that gives you great confidence even though we haven't had the opportunities to play our best team.

"I think that's the way we're building. And we know that, come the World Cup, we will have the opportunity to pick from everyone who's available."

AFP