Former England captain Terry Butcher believes Roy Hodgson's defenders will only improve by training in an attack-minded squad.
The 57-year-old, who made 77 England appearances, has reservations about a defence where Chris Smalling, Gary Cahill and John Stones are the only recognised centre-backs for Euro 2016.
But Butcher reckons England's defensive resilience will improve by training against the likes of Harry Kane, Jamie Vardy, Wayne Rooney and Daniel Sturridge.
"It's an exciting squad, one of the youngest squads England has ever produced," Butcher said.
"It's based on attack, going forward, scoring and creating goals.
"The best form of defence is attack. At the back it still worries me.
"We give away some silly and easy goals. There doesn't seem to be a real resilience, a real steely determination not to concede.
"It seems to be just a case of hoping that teams don't score, rather than saying they definitely won't.
"That will come. We found that in tournaments that you got more understanding, more determination, more spirit when you're together all the time.
"The thing about England in training is that the defenders are up against very good attack-minded players in the squad.
"We always found training sessions extremely hard, because you're up against really class players. That's the same now with England.
"England's defence - if they can keep out England's attack and midfield players - then they won't face many more difficult opponents in the tournament itself."
Despite England struggling to overcome 10-man Portugal at Wembley in their final friendly before Euro 2016, Butcher believes Hodgson's side can do well in the competition.
"I'm optimistic. I think they can go through to the last four," he said.
"Spain are the holders and Germany the World Cup winners, but England have played those two teams and know what they're up against.
"There shouldn't be any real worries for England about the opposition. It's a case of what England do themselves, more importantly."
Butcher was speaking ahead of taking part in his second Prostate Cancer UK Football to Amsterdam cycle challenge, which began on Friday.
Butcher said: "I enjoyed it so much last year. The cause is a fantastic cause.
"It's a horrible illness. We all know people who have it or have had it. If you're going to do one event of the year, this would be the one to do.
"It encapsulates the whole spirit of football fans as well as everyone having a good time."
Butcher's former England team-mate Viv Anderson has been persuaded to take part, despite almost 30 years off the bike.
Anderson, who has been training since January for the event, said: "I had a disc removed when I was playing for Manchester United in 1987, 1988. I had to cycle to work every morning.
"I vaguely remember my bum was so sore I had to strap foam around the seat.
"Last time I was on a bike, you put your foot on the pedals and off you go.
"Getting used to gears, Lycra... it's a completely different world to what I was used to.
"I've just got to get to Amsterdam now."
:: Prostate Cancer UK, the Official Charity Partner of The Football League, will stage the Football to Amsterdam cycle challenge from June 3-5, 2016 with riders now able to roll out from London and two new starting points in Yorkshire, Oakwell home of Barnsley FC and Ipswich Town's home ground, Portman Road. For more information about the 2017 ride, visit prostatecanceruk.org/amsterdam or contact the cycling team by calling 020 8222 7158 or emailing email@example.com.