Dozens of people in and around Leicester have reported hearing an explosion or a sonic boom overhead.
There are suggestions it was heard as far away as Oxford.
The loud bang led to a swift reaction on social media as people speculated about the cause, including whether it was a jet breaking the sound barrier.
Twitter user Dr Jon Sutton said: “Massive boom heard over a wide area of Leicestershire. Aircraft, or meteor?”
Naomi tweeted: “Anyone in Leicester know what the hell that massive explosion was?? It shook our house and the birds are going crazy.”
There are reports it was a RAF Typhoon aircraft scrambled to intercept a plane that had suffered radio failure, which later landed at an airfield in Epping.
Leicestershire police said: “We have received numerous calls in relation to a large explosion sound heard from various parts of the city and county.
“We like to reassure you that there is no concern however thank you for your immediate response to us.”
The neighbouring Northamptonshire force said: “Police and fire services in Northamptonshire have received numerous calls in relation to a large explosion sound heard from various parts of the county.
“We would like to reassure people there is no concern, but thank you for contacting us.”