Sussex will promote from within to fill their vacant head coach position after Jason Gillespie's departure from the club, with current assistant coaches Ian Salisbury and James Kirtley splitting the role across formats.

Sussex will become the third county after Derbyshire and Middlesex to have employed a specialist T20 coach, with Kirtley, a Twenty20 Cup winner with the club in 2009, taking on the job for the Vitality Blast. Salisbury, who played 15 Tests for England as a legspinner, will be in charge for the County Championship and the One-Day Cup.

Sussex are a club in transition, having lost several senior players over the last few months. Laurie Evans and Danny Briggs have moved to Surrey and Warwickshire respectively, while David Wiese is unlikely to return - unless as an overseas player in the Blast - after the expiration of his Kolpak registration. Luke Wells, the opening batsman, is in discussions with other counties after the end of his deal, while Harry Finch and Will Sheffield were both released.

It is understood that Sussex made contacts with some external candidates for the job, including Warwickshire's sport director Paul Farbrace, but they instead decided to promote from within. The club announced "a limited number" of redundancies last month and a reduction in playing budget amid a £3 million loss of income.

"As we head into the winter with so much uncertainty, we wanted to give clarity to everyone involved with the club," Rob Andrew, Sussex's chief executive, said. "Ian and James have worked well together both here and elsewhere, are passionate about the club and have a clear vision for all formats.

"The county game will evolve further next year with the launch of the Hundred, and this is a move taken with that in mind. We have developed a strong T20 team and now have several players who focus on white-ball cricket only. We believe that is a trend that will continue to grow in the future and we need to be able to cater for this.

"The Royal London Cup will be played during the Hundred, when we will have players missing. This will enable us to continue to develop our young players during the 50-over competition. Our four-day form has been disappointing for a few years and we have a clear strategy to build a successful team over the coming years, primarily with home-grown players and high-quality overseas where appropriate."

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Salisbury said that he was "immensely proud, honoured and humbled" to have been awarded his role. "The chance to work in unison with James is one I'm hugely excited about," he said. "Our shared vision and strategy is to bring sustained success to Sussex. We have the talent within the squad and the potential talent coming through from the most impressive academy in the country to achieve this."

Kirtley said: "I am thrilled to have been given this opportunity with the T20 team and very grateful to the club for entrusting me with this role. I am very much looking forward to working with Luke Wright [T20 captain] and the rest of the squad in delivering silverware to this club.

"It will be exciting to help find the next level with the T20 side and with the help of the coaching staff, senior players and analysts I am confident we will be successful in discovering the small margins and making the changes necessary to take us further in the competition."

Keith Greenfield, Sussex's performance director, said that he expected to see more counties using different coaches for different formats going forward. "This is a great opportunity with us," he said, "with split coaching roles a proactive new course for the club and one that you will see others follow in the future."