According to a report by The Sun on Monday, Chelsea centre-back Andreas Christensen is moving ever closer to a move to Barcelona on a free transfer.

The 25-year-old is out of contract at Stamford Bridge this summer - and is understood to have already agreed personal terms in principle at Camp Nou, having previously rejected Chelsea's offer of a new five-year deal to stay in west London.

Bayern Munich had also been linked with a swoop for the Danish international, but it appears increasingly likely that La Liga will be Christensen's next destination after he completed the first part of his medical with Xavi's side last week.

The remainder of that process is set to be undertaken this week, after which Barcelona and Christensen could technically announce the deal.

If Christensen does land in Catalonia, though, he is likely to be far from the only big name to be involved in a blockbuster free transfer this summer.

LONDON, ENGLAND - DECEMBER 28: Andreas Christensen of Chelsea in action during the Premier League match between Chelsea and Aston Villa at Stamford Bridge on December 28, 2020 in London, England. (Photo by Richard Heathcote/Getty Images)

The amount of top-tier talent that will be out of contract at the end of June this year is staggering, with some genuine superstars of the game eligible to leave their current clubs without a penny being required in terms of a transfer fee.

With the help of Transfermarkt, we've taken a look below at the 20 most valuable players in the world who are out of the contract before the start of next season - and a few sides set to lose some seriously valuable assets for nothing.

The 20 most valuable players who are out of contract this summer

20. Florian Grillitsch (Hoffenheim | £14.4 million)

19. Luiz Felipe (Lazio | £16.2m)

18. Andrej Kramaric (Hoffenheim | £16.2m)

17. Noussair Mazraoui (Ajax | £18m)

16. Jason Denayer (Lyon | £18m)

15. Alessio Romagnoli (AC Milan | £18m)

14. Jesse Lingard (Man Utd | £18m)

13. Alexandre Lacazette (Arsenal | £18m)

LONDON, ENGLAND - OCTOBER 18: Alexandre Lacazette of Arsenal celebrates after scoring their team's second goal during the Premier League match between Arsenal and Crystal Palace at Emirates Stadium on October 18, 2021 in London, England. (Photo by Catherine Ivill/Getty Images)

12. James Tarkowski (Burnley | £19.8m)

11. Matthias Ginter (Monchengladbach | £21.6m)

10. Boubacar Kamara (Marseille | £22.5m)

9. Andrea Belotti (Torino | £25.2m)

8. Ousmane Dembele (Barcelona | £27m)

7. Andreas Christensen (Chelsea | £31.5m)

6. Antonio Rudiger (Chelsea | £31.5m)

ABU DHABI, UNITED ARAB EMIRATES - FEBRUARY 12: Antonio Ruediger of Chelsea celebrates with The FIFA Club World Cup trophy following their side's victory during the FIFA Club World Cup UAE 2021 Final match between Chelsea v Palmeiras at Mohammed Bin Zayed Stadium on February 12, 2022 in Abu Dhabi, United Arab Emirates. (Photo by Francois Nel/Getty Images)

5. Marcelo Brozovic (Inter Milan | £36m)

4. Franck Kessie (AC Milan | £43.2m)

3. Paulo Dybala (Juventus | £45m)

2. Paul Pogba (Man Utd (£49.5m)

1. Kylian Mbappe (Paris Saint-Germain | £144m)

PARIS, FRANCE - SEPTEMBER 28: Kylian Mbappe of Paris Saint-Germain warms up prior to the UEFA Champions League group A match between Paris Saint-Germain and Manchester City at Parc des Princes on September 28, 2021 in Paris, France. (Photo by Shaun Botterill/Getty Images)

Chelsea could feasibly lose both Christensen and centre-back Antonio Rudiger this summer and their combined value still wouldn't come to half of the £144m that PSG are set to miss out on at present if French forward Kylian Mbappe walks away from the Parc des Princes for nothing at the end of June.

The Ligue 1 giants reportedly turned down a near-world record offer from Real Madrid to part company with Mbappe last August, but turned it down in the hope that they could convince him to sign a fresh deal at the club.

As of yet, an agreement hasn't been reached - and time is beginning to run short for the Parisiens.

MANCHESTER, ENGLAND - FEBRUARY 26: Paul Pogba of Manchester United gestures during the Premier League match between Manchester United and Watford at Old Trafford on February 26, 2022 in Manchester, England. (Photo by Nathan Stirk/Getty Images)

Elsewhere, Paul Pogba's future at Manchester United has been the subject of speculation for quite some time. Despite the best efforts of the Red Devils, the 28-year-old hasn't committed to extending his stay at Old Trafford.

His United colleague Jesse Lingard, meanwhile, is also out of contract and appears likely to leave the 'Theatre of Dreams' in a few months.

Other Premier League players who could head for pastures new without the need for a transfer fee include Arsenal striker Alexandre Lacazette and Burnley defender James Tarkowski.

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Finally, while Barcelona appear to be in pole position to snag the services of Christensen from next season onwards, La Blaugrana also face losing a pretty valuable player themselves, with unhappy winger Ousmane Dembele shortly free to depart the club for free.

Of course, each of the players featured on the above list could still choose to ink a contract extension with their current employers.

We don't expect that to happen in too many cases, though, as the footballing world braces itself for a flurry of transfer activity over the summer months.

City destroy United in Manchester derby (Via The Football Terrace)