John Sheridan has resigned as Carlisle United manager in sensational fashion - and is expected to join former club Chesterfield.

The Blues announced the shock departure of the 54-year-old this afternoon.

The news comes with United currently in the League Two play-off positions and having won all five of their matches over the festive period.

Assistant manager Tommy Wright and first-team coach Paul Murray will take caretaker charge for tomorrow's game against Mansfield, assisted by director of football David Holdsworth.

Carlisle say the process of finding a new boss has started immediately.

In a club statement, chairman Andrew Jenkins said: "The request to leave came from John.

"Over the last few days, the board and John had discussed football squad matters, and these were being resolved. I would like to thank him for his commitment, and we all wish him the best of luck for the future."

Jenkins added: "In his short time at United, John has made a positive difference to the club.

"He has improved the style of football and assembled a squad that is able to compete with the best teams in the League Two. 

"Working with the squad and management team, he has helped to establish a winning formula at the club that we will take forward and build on to ensure that John's hard work is not lost."

United did not comment on reports earlier today that suggested Sheridan was poised to return to Chesterfield.

He has been odds-on favourite for the position at the National League strugglers in recent days.

His future also came under the microscope after he used a post-match interview on Boxing Day to threaten to quit if people could not cope with his "demanding" style.

That followed some heated exchanged with players including captain Danny Grainger during that day's game - a 6-0 win against another of his former clubs, Oldham.

Sheridan previously managed the Spireites from 2009-12.

He was appointed as United manager in the summer and has taken charge of 32 games in league and cup.

His departure comes at a crucial time for the seventh-placed Cumbrians, who are in talks with transfer targets and hoping to keep existing players such as Anthony Gerrard, who has been offered a new deal and is a player close to Sheridan.

The future of loanees Ashley Nadesan and Jack Sowerby are also yet to be clarified this month.