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After England Under-17s won the World Cup, the spotlight is again on how best to develop young footballers into future stars.

There are an estimated 3,000 children aged between nine and 16 in Premier League academies, with thousands more throughout the Football League.

BBC Radio 5 live Investigates has looked at the academy system in English football, including the story of a 15-year-old who is looking for his fifth club - but has a £120,000 'transfer fee'.

'This could be the end of my career'

"Unless you're the next Cristiano Ronaldo, clubs don't want to pay £120,000 for a 15-year-old."

Zac Brunt has been part of academies at Aston Villa, Manchester City, Atletico Madrid, and most recently Derby.

He spent the past two years with the Championship club's elite academy on what is commonly known as an Elite Player Performance Plan (EPPP) - designed to bring the best young players, the best coaches and the best environments together at an early age.

"I felt like I wasn't getting anywhere at Derby," the 15-year-old told the BBC. "Other clubs are interested but the fee is so big. For two years' work, have I cost Derby County £120,000? Probably not.

"I think this could be the end of my career. I can't go anywhere else apart from semi-professional - the highest level I can go - unless a professional team buys me out of this clause."

Derby told BBC Sport it would be "inappropriate" to comment on the "development or performance of individual players". The club added it "prides itself on class-leading pastoral care" and the quality of its academy, and it "retains the right to seek compensation" for any player who leaves.

The Premier League and the English Football League (EFL) said the "vast majority" of young players moving between academies have compensation waived.

Why so much money?

Compensation formula for academy players when joining another club Age group of player Category of club academy leaving Applicable annual fixed fee - new club to pay U9 to U11 All Categories £3,000 U12 to U16 Category 1 £40,000 U12 to U16 Category 2 £25,000 U12 to U16 Category 3 £12,500

The £120,000 compensation fee that any other academy must pay Derby to sign Brunt is worked out from his three-year registration at their category one academy (£40,000+£40,000+£40,000).

Before this agreed system was in place across professional English football, young players would move between academies and a tribunal would agree a fee.

Brunt says he understands the need for a rule to protect clubs' investments, but added: "I know a few boys who have been absolutely ruined by this rule and they had to stop playing football and go and play non-league or something like that because they just can't get in anywhere.

"The time and effort the clubs put in is not as much as the kids put in - it's just the minority of the kind of effort that I put into football."

Brunt's dad Glen says his son has been for trials with other top clubs, but the compensation fee is putting them off - comparing it to the £7,000 Derby paid Manchester City to sign the youngster.

"In theory, his football career could be dead at the age of 15 - and that may still prove to be the case," he said.

"It's all about the coffers - banking the money - and there is no thought whatsoever for the child and the family stuck in the middle.

"A lot of people would strike me down for the lack of stability in his football - but we are not willing to have him trained in a certain way.

"He's strong minded himself and he makes these decisions himself for the past few years.

"It's become difficult now where a club has to be beyond doubt that he's going to be a top player for someone to pay £120,000. That's the harsh reality."

Zac (pictured with his dad Glen) feels his career could already be over - "Unless you're the next Ronaldo, clubs don't want to pay £120,000 for a 15-year-old"

The compensation system explained

How it usually works

A player and their parents sign registration forms to join a Premier League club - that includes a compensation system, which works as follows:

Boy A signs for a Premier League team for two years.

Parent of Boy A gets email from the Premier League with Charter for Parents and compensation explanation.

At the end of two years, boy turns down a new offer from the current team and joins a new club.

The new club pay the old team a compensation fee for the two years of training and development.

This helps protect smaller and mid-tier clubs and enables them to invest in their academies with some form of protection.

The YD10 form scenario

Separately, if Boy A decides to leave a Premier League club's academy early - before their registration period has ended - a YD10 form is signed.

This gives the club the right to retain compensation for the years they have trained and developed Boy A.

If Boy A wants to then join another club, the new club must pay the original club compensation.

The Premier League says in the "vast majority" of cases where this happens, the original club waives the right to future compensation.

A copy of a YD10 compensation form that a player & parent sign when leaving an academy early

So is this an unusual case?

The Premier League and the EFL say so.

For example, from about 3,000 Premier League academy players last season, 210 left a club early and, of those, 182 (87%) had compensation rights waived. That percentage has remained above 80% for the past three years.

The EFL also says in the past 12 months, in over 90% of cases where a player has left an EFL club's academy, there has been no claim for compensation.

The Premier League said in a statement:

All parents are contacted by the Premier League when a player joins an academy - with the Charter for Parents and information on the "balanced and proportionate compensation system".

"Clubs would have little incentive to invest in providing the best possible environment in their academies if their young players could move freely at any time without recognition of the years of coaching and education they have received."

Parents can "mutually agree" with the club to use a YD10 form when leaving an academy early. "In some cases, should they join another academy, it requires their new club to pay compensation for the years of training and development the player has received at their previous club."

"In the vast majority of cases involving a YD10 form, Premier League clubs waive any rights to future compensation."

An EFL spokesman said:

The rules - in place since 2010 - represent a shared view that protects players and clubs.

They are in place to support clubs' efforts to invest in youth development.

Registration rules are explained to players and parents.

Players can apply for their release free from compensation - and that is considered by independent panel.

In a statement to the BBC, the Football Association said: "As the governing body of the game, player welfare is of paramount importance to us.

"The FA works collaboratively with the Premier League and EFL, both which have rules in place to protect young players."

'Some parents don't realise what they are getting into'

Shaun Daly was a scout at Chelsea and a coach within their development programme before becoming head of recruitment at Tottenham for players aged 6-12.

He is now head coach at Focus Football, a soccer school in north London where players can play and train after leaving or being released from an academy.

When it comes to the compensation fees in contracts, he said: "A lot of the time I think when the players are so young some of the parents are just so happy to be given a contract that's signed.

"They don't actually think of reading it.

"If they were going to sign up for a two-year phone contract they'd read every little part of that contract, but because Tottenham, Arsenal, Chelsea, Manchester United, Barcelona come and go 'we want to sign you' they are just so grateful at that moment and don't realise what they are going in to.

"Some clubs I'm sure explain it well. Some others don't as clearly."

'It's a big problem'

Dan Chapman, a sports lawyer and FA registered intermediary, says: "I have been consulted by more players in this situation than they allege to have sought compensation to be waived.

"It's a big problem. The Premier League has admitted it sends parents the rules after they have signed up so what good is that?

"From a legal perspective, at some point, we need to challenge this because that's just an absurd proposition legally.

"Sadly, a lot of parents will have never seen it at all. I'm sure some clubs do explain it and that must be right but the vast majority of parents that consult me only find out about the compensation system after it is too late."

Former footballer Jean-Marc Bosman famously took his case to the European courts and, in December 1995, won - setting a precedent allowing footballers to leave their club for no transfer fee once their contract had expired.

Chapman added: "Next summer, we will probably see the most famous Bosman transfer in the history of football - by all accounts Alexis Sanchez will leave Arsenal on a free transfer.

"Yet, a nine-year-old who might one day aspire to be a Sanchez, but in all probability has no realistic prospect of that, carries a compensation charge.

"He doesn't have freedom of contract, Sanchez does. It can't be right."