Youngs retired from international rugby at the end of the latest tournament, while Farrell, 32, has stepped away for the 2024 Six Nations to "prioritise his and his family's mental wellbeing".

"He's a fantastic man," Youngs said.

"He is without doubt the best captain I've played under. He is brilliant. Everything he has done is fully committed to England, to helping the team, to being the best he can be. But also helping the team be the best they can be."

Youngs, who has spent his entire club career with Leicester Tigers, has played under more than half a dozen England captains since making his senior debut in 2010.

Farrell was in the centres and Youngs was starting scrum-half as England were beaten by South Africa in the World Cup final four years ago.

And Youngs started alongside fly-half Farrell in his 127th and final appearance, as England beat Argentina to collect World Cup bronze in France in October.

Their head coach was Steve Borthwick, who was England captain when Youngs emerged more than a decade earlier.

It was during the tournament that Farrell became the country's all-time record points scorer, surpassing Jonny Wilkinson's career haul of 1,179 points.

But Saracens star Farrell, who was suspended for England's opening two World Cup games for a dangerous tackle during a warm-up win against Wales, was also booed by some supporters during the tournament.

Youngs said Farrell's treatment was "totally unacceptable".

"I think he has been so unfairly treated," Youngs told BBC Radio Leicester.

"He should be treasured and celebrated as a person and as a player for England and not have to deal with the stuff he had to deal with.

"He is someone that has given everything that he can for his country and done a tremendous job.

"He should be celebrated and I think it is really unfortunate that he wasn't and that he had to deal with the situation he had to deal with.

"From just a rugby point of view, not from a friend's point of view, I think it's really sad."