Semley, pictured, left the Vikings almost a year after leaving Nostell Miners Welfare to take over from Curtis Woodhouse.
His exit in the wake of Saturday's 2-2 draw at home to Tividale followed a winless nine game run that has left the club hovering one place and a single point above the First Division South drop zone.
In a statement announcing the departure of the former Barnsley striker, officials said they hoped to make a swift appointment with the EVO-STIK NPL's transfer window shutting soon and invited 'suitably' experienced potential successors to send their footballing CVs to club chairman Steve Shorthouse.
Shorthouse also paid tribute to the former reserve team boss at Emley, Frickley and Ossett Town and the work he has done since taking charge at the Victoria Pleasure Grounds.
The chairman said: "The decision was a very difficult one. Alan joined us last January with only four players available to him, with a very small budget to play with and was instrumental in playing his part in the club's survival.
"This season started off very well, seeing us as high as sixth in the league at one stage. Unfortunately, no wins in nine games sees us very much in a relegation dogfight and the board decided that a change of manager was needed now, especially with the league's transfer window closing soon.
"I would personally like to extend my thanks to Alan and Keith for all the hard work they have done. I am bitterly disappointed for both of them and wish them both well for the future."
Former Guernsey boss and Vikings veteran Kevin Graham will take charge of the first team on Tuesday night when they head to promotion chasing Shaw Lane Aquaforce.
Applications for the post should be sent via email in strick confidence to: shorty25558@icloud.com.
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