Spanish clubs Barcelona and Real Madrid have joined Chelsea and Manchester City in the battle to sign John Stones. The 21-year-old Everton central defender is seen as one of the most promising defensive prospects in the U.K. and has attracted the interest of several clubs across Europe. This summer, Stones will be at the center of a £50 million scramble, according to Jonathan Northcroft of The Sunday Times.

The Toffees already turned down requests from other teams to sign their prized center last summer when they rejected a £38 million offer from Chelsea. However, the interest in Stones has heightened since then, with former Real Madrid manager Rafa Benitez asked to sign him. Although Benitez has parted ways with the club, Real Madrid have maintained their interest in Stones.

The Everton defender is just one of Madrid's transfer targets. The team may plan to sign Stones in the summer, but it is still currently banned from signing new players. Real Madrid is reportedly planning to appeal the ban and see if they could have it delayed until next year.

"It's an unprecedented situation for an English player to be at the center of quite so many of the world's top clubs' attention," Northcroft told Sky's Sunday Supplement. "He's on Real Madrid's shopping list; though whether they'll be allowed to do transfers in the summer remains to be seen. But he's on their list of targets alongside Robert Lewandowski and David Alaba."

Back in January 2013, Stones joined Everton from Barnsley in a deal said to be worth £3 million. He still has three years remaining on his contract. The Liverpool-based English club may prefer to move Stones abroad rather than strengthen another Premier League club.

In more Everton transfer news, the Toffees seems to be intent on fending off interest in Belgian striker Romelu Lukaku by placing a huge price tag on his head. According to The Sun, Everton informed other clubs that any bid below £65 million will not be considered.

Lukaku already delivered 19 goals this season in all competitions. The 22-year-old goal machine has attracted the attention of quite a long list of European teams including the Manchester United, Real Madrid, PSG and even his former club Chelsea. Old Trafford is said to be most keen on acquiring the Everton striker and have been monitoring him closely this season. It remains to be seen whether the expensive asking price will put off Manchester United manager Louis van Gaal's side.