England number eight Billy Vunipola trained on Tuesday and is "very likely" to start Saturday's Rugby World Cup quarter-final against Australia, according to assistant coach John Mitchell.

The Saracens man twisted an ankle in the pool-stage win over Argentina and would not have been risked at the weekend had England's clash with France gone ahead, rather than being cancelled due to Typhoon Hagibis.

Vunipola has returned to full training, however, and Mitchell is confident he will be fit for the clash with the Wallabies in Oita.

"Billy is progressing really well," he said. "He has trained again today so we are very confident in progression each day.

"He is a very important player to us and a very likeable player as well. He fits well within the team."

Asked to rate Vunipola's chances of playing, he added: "Very likely."

England had another observer at training with Australian rugby league legend Ricky Stuart invited to attend by head coach Eddie Jones.

Stuart's involvement has raised eyebrows in the Australia camp, not least from their coach Michael Cheika, but Mitchell is confident England's players will benefit.

He said: "Ricky and his coaching group have just arrived today. It's great to see them again.

"One of the great things I believe Eddie does in our environment is encourage a learning environment.

"Ricky is not the only coach from rugby league or any other sport that has come in, we have them on a regular basis. We want to see what we are doing can be improved and we like to learn off others and that's the great opportunity we have.

"It's just a watching, learning and sharing process that occurs, to get somebody in we can share and learn off and create a stimulus around. They've just recently played in one of the major rugby league competitions in the world [NRL grand final], you'd be stupid if you weren't able to gain something from that."

Mitchell, who has coached in Australia with the Western Force, is expecting a tough game against Cheika's men.

"They will be clever at the weekend," he said.

"They are always clever and always have the ability to surprise. They love ball in their hands which is something which they thrive on."