England captain Owen Farrell has adjusted his tackle technique in fear of falling foul of World Rugby's crackdown on dangerous play.

Farrell was guilty of reckless no-arms challenges against South Africa and Australia last autumn – escaping punishment for both – to raise concerns over his risky style of halting opponents.

World Rugby are determined to rid the game of contact to the head and in May issued a directive clarifying the process for officiating high tackles and shoulder charges, including the wider use of cards.

While England head coach Eddie Jones is concerned that games risk being "destroyed" by a poor decision from officials, he has also seen the value in refining Farrell's approach.

"Owen's not overly focused on that area, but he has made some adjustments," defence coach John Mitchell said.

"John Carrington, our strength and conditioning coach, is also my support on defence and he's very good at working on tackle technique. They have made some adjustments based on that (last autumn).

"There can be mitigating circumstances, but if your hands are in front of your shoulder you've got a better chance of making a proper wrap tackle.

"If your shoulder is ahead of your hands, then the law doesn't look after you very well."

The impact of the drive to improve player safety was visible at the recent World Under-20 Championship in Argentina which produced a total of 13 sin-binnings and four dismissals.

Mitchell insists every team will enter Japan 2019 with a sense of uncertainty over how the officiating will play out in practice.

"It's at the back of everyone's mind and there is some apprehension around it," Mitchell said.

"You are just going to have to deal with whatever happens. There will definitely be some apprehension around – 'OK, is it actually going to be like that?'.

"You'd like to think that since then there has been learning, as little bit more common sense and you do hear the words 'mitigating circumstances'."

England open their World Cup against Tonga in Sapporo next Sunday in a fixture that is sure to keep their medics busy.

The Pacific Islanders are famed for their bone-jarring hits in defence and Mitchell insists they could suffer at the hands of referees if they have not done their homework thoroughly.

"They like a tackle and it's something that they get excited about," Mitchell said.

"But it's good for the game in the way we now have a good calibration to look after the head, so it's really important that you have to adapt.

"The sides that haven't spent time on tackle technique and height and have left it to remain liberal, that could become costly.

"It's certainly an important part about our process and we make sure that we train it in the way that it'll give us an advantage, but ultimately we understand the head is a no-go zone."

Joe Cokanasiga and Mark Wilson have emerged as doubts against Tonga due to knee problems.

Cokanasiga and Wilson have been unable to take part in training at the squad's camp in Miyazaki due to the injuries and according to Eddie Jones they are now undergoing a "mini pre-season".