Pompey, Oxford United, Peterborough United and Carlisle United set to leave next summer - as things stand: Photo gallery

The January transfer window is just over a month away, and that represents just over the half-way point of the season.

Pompey lead the League One standings and are currently unbeaten with 10 wins and six draws. John Mousinho's done a good job since being appointed to the Fratton Park hot seat, and in the summer he was allowed to sign more than 10 players. He's not just working with players he's signed however, and there are players who were signed under previous managers such as Danny Cowley.

Players will have six months remaining on their contracts, and there will be different situations. Some players want their futures finalised now, whilst others are happy to wait and see how the campaign pans out. In the third tier of English football, there aren't as many clubs who are able to offer a contract of more than three-years unlike Premier League and Championship clubs because of the finances involved. The teams who can however usually have bigger resources, and ultimately end up getting a nice fee for their players.

For Portsmouth, they will be hoping that they're in the Championship next season. It's been too long since they graced the second tier of English football, and even longer since they were in the Premier League, winning FA Cups and playing AC Milan in the UEFA Cup. Should they manage to see how out the job, there are some players that could be available on a free transfer that could be of note.

Flick through to see 25 of the most valuable players in League One whose contracts expire this summer. All information is based off of what TransferMarkt provides.