Shortly after Jeremy Corbyn was elected leader of the party in September, Slocombe was taken on board as interim spokesman to the leader, although the party was not initially able to confirm how long that interim hire was for.
Slocombe has now told PRWeek that he has taken on the position of head of media for the leader of the party, effective 24 November.
He also said that a new spokesperson for Corbyn would be "announced soon".
Prior to working under Corbyn, Slocombe was head of comms for postal and telecoms union the Communication Workers Union, He spent nearly 10 years at the organisation before leaving in June.
Corbyn, who spent many years as a backbench MP before his rise to leadership, has faced questions over his PR and his ability to handle the media since the start of his tenure.
In October, the party announced the hire of Guardian columnist and associate editor Seumas Milne as executive director of strategy and comms.