It would be a "national disgrace" if clubs stopped England's best youngsters playing at the 2015 Under-21 European Championship, says Gary Lineker.
The U21 side beat Germany 3-2 in a warm-up for the summer event, which is being held in the Czech Republic.
Lineker thinks England could go on to win the Euros - but only if they have a full-strength squad to pick from.
Harry Kane and Ross Barkley could both feature for the U21s but there are fears their clubs could block call-ups.
Spurs are due to play Sydney FC in Australia at the end of May, less than three weeks before England's first U21 championship game against Portugal on 18 June.
Everton manager Roberto Martinez said recently that midfielder Barkley would need a "proper break" in the summer.
"If clubs try to pull their players out, it would be a national disgrace," tweeted ex-England striker Lineker. "England beat Germany. Add a few from the seniors and they could win Euros."
Former England, Manchester United and Everton player Phil Neville told BBC Radio 5 live he agreed with Lineker.
"Harry Kane's development as a player will only be enhanced by going to the tournament in the summer," added Neville.
Kane, who has scored 29 goals for Spurs this season, helped the U21s qualify for the Euros but was recently promoted to the senior squad.
England then have a friendly against the Republic of Ireland on 7 June, followed by a Euro 2016 qualifier in Slovenia on 14 June.
England boss Roy Hodgson believes Kane should play for the U21s in the Czech Republic, rather than be part of the senior set-up.
However, he suggested last week that Tottenham's commercial interests could stop Kane from playing for the U21s.
"We still feel we can give the players the best possible preparation to go into the seniors and achieve success if they have got the experience of the European Under-21 Championship behind them," said U21 coach Gareth Southgate. "That is still the case with Harry."
Southgate has already resigned himself to being without Liverpool winger Raheem Sterling.
"Realistically, Raheem has become a very important player for the senior team and for Liverpool," said Southgate. "I'm not convinced another tournament in the summer would be right for him."
Lineker says he is "baffled" by the stance Premier League clubs have towards the international game and believes a successful national side would have a "massively positive impact" on English football's top flight.