London - Alex Corbisiero believes England successor Mako Vunipola is now the "best loosehead prop in the world".
Vunipola has been in fine form for English and European champions Saracens so far this season, with his work at the set-piece -- once a source of criticism -- as well as in open play winning impressive reviews.
The injury-hit Corbisiero, who started a sabbatical from rugby last year after winning 20 England caps and two for the British and Irish Lions, was in no doubt of Vunipola's all-round quality.
"In my opinion Mako is the best loosehead in the world and I will defend that view against anyone. He's moving into a class of his own," Corbisiero said.
"His set piece has become so consistent and it's ever-improving. He's becoming a dominant scrummager.
"Everything he does around the field is incredible - his work-rate, his defence, his turnovers, his carrying, offloads, passes out the back."
Corbisiero, speaking at an event to launch the N-pro head guard, which aims to reduce the risk of concussion, added: "He's never in the wrong position and is a great, intelligent rugby player who makes the right decisions who can make an impact while covering the basics that a prop needs to do."
England scrum coach Neal Hatley said the set-piece game of Vunipola, whose brother Billy is Saracens and England's No 8, had come on leaps and bounds.
"Mako has been absolutely outstanding for Saracens in the first half of the season," Hatley said.
"There aren't many props in world rugby that can do what he does with the ball in hand.
"His set-piece has come on immeasurably.
"Sometimes people get pigeon-holed as a carrying prop or a scrummaging prop and I think he crosses that divide.
"He's been very sound from a set-piece point of view. We've made a few little technical changes to his bind and that's had a real positive effect.
"There are a lot of good players around, but he's definitely getting there in terms of being the best loosehead in the world. If Corbs said that, it's a pretty good endorsement."
England open their end-of-year international programme against a struggling South Africa at Twickenham on Saturday, with the Springboks having lost five of their last nine Tests including a record 57-15 defeat by world champions New Zealand in Durban last month.
But former London Irish prop believes South Africa, renowned for their physical approach to rugby union, will still pose formidable problems up front for an England side looking to make it 10 wins out of 10 under Australian head coach Eddie Jones.
"They will probably go with a big pack and a big back five in the way they've tried to do for most of the year. It will be a good side," Hatley said.
"They have been a state of flux, but when they've played well they have been very impressive. The Lions did well in Super Rugby. We're expecting a good side to turn up.
"They have a very good scrum and will probably load up on that back five. There will be big bulk there so it will be a testing day from a scrum point of view."
Both England and South Africa are due to name their teams on Thursday.