The account in the Lithuanian press I posted above is confirmed in the London Evening Standard
A spokesman for Ryanair said: "This flight from Kaunas to London Luton (4 Oct) was delayed ahead of take-off after Lithuanian authorities received a suspected hoax security alert.
"Police carried out a security search and cleared the aircraft to return to service, customers boarded and the flight departed to Luton.
While on approach to Luton, upon the request of UK authorities, the aircraft diverted to London Stansted in line with security procedures, and landed normally at Stansted, where customers disembarked and were transferred to Luton by coach."