A Jet2.com Boeing 737-300, registration G-CELI performing flight LS-509 from Newcastle,EN (UK) to Prague (Czech Republic), was enroute at FL310 about 90nm northnorthwest of Frankfurt/Main (Germany) when the crew initiated an emergency descent due to the loss of cabin pressure, the passenger oxygen masks were released. The aircraft diverted to Frankfurt for a safe landing on runway 25C about 25 minutes after leaving FL310. The airline reported the crew performed a controlled descent following a technical fault indication. A replacement aircraft was dispatched to Frankfurt. A replacement Boeing 737-800 registration G-GDFX positioned to Frankfurt and reached Prague as flight LS-509A with a delay of 5 hours.