Sir Clive Woodward believes England players are lacking the required level of fitness for Test rugby, saying "they've been going backwards for the last 13 years".
Woodward, who led the English to World Cup glory in 2003, is shocked by their current conditioning and can't understand the reasons why.
In his column for the Daily Mail, the former England head coach agreed with current defence coach Paul Gustard that work needs to be done.
"I did not know whether to laugh or cry when England coach Eddie Jones and his defence man Paul Gustard both insisted this week that England weren't fit enough," he said.
"This was nothing new to me. My 2003 team were coached and selected on the basis of being 'the fastest and fittest team in world rugby', which we became.
"In recent years there has been a total misunderstanding of what is physically required to be the No 1 team in the world. This is a power sport but at the highest level that isn't enough. If a team don't have the ability to relentlessly attack and play at speed for 80 minutes you're a sitting duck and you'll be blown away by those who can."
Woodward added: "Looking man for man between my England team in 2003 and the England team at last year's World Cup, I would still rate my team as being fitter and better equipped physically to beat the speed of the southern hemisphere teams, which is a staggering statement to make.
"How can that be, given all the advances in sports science we read about? And who has been responsible for that decline?
"England haven't just been standing still, they've been going backwards for the last 13 years."