Bus fleets in the Tees Valley and across the UK need to reduce their carbon footprint and emissions. New hydrogen fuel-cell powered buses offer a solution, however dealing with the existing fleet of vehicles is a significant challenge for the industry. The UK bus and commercial vehicle fleet operators need an affordable, ecological, quickly deliverable solution to transition the existing fossil Diesel powered fleet to zero-emissions. Using Ricardo's deep technical knowledge and capabilities for clean efficient propulsion systems (including fuel-cell/battery system powertrain) and Stagecoach's expertise in bus operations, the project HYRBUS (HYdrogen fuel-cell Retrofit kit for BUS) is developing a bus demonstrator with a hydrogen fuel-cell powertrain solution that can be retrofitted to existing Diesel buses. The demonstrator will be driven in the Tees Valley area on test grounds and public roads from Mid February 2022 to end March 2022\. Demonstration trails will continue from 1st April to mid May 2022 in Tees Valley with driving on public roads. The bus will use regularly the hydrogen refuelling station in Tees Valley area. Subject to success for initial 3 months of demonstration activities under the competition scope, the demonstration trials would be expected to continue, with an expanded fleet, for up to 6-9 months to prove performance, reliability, maintainability and efficiency of the retrofit solution. After this, the intention is to make the hydrogen fuel cell retrofit solution available as a commercial conversion service sold by Ricardo.