Calculations show that the tank would have begun to overflow at about 05:20.[1][10] There is evidence suggesting that a high-level switch, which should have detected that the tank was full and shut off the supply, failed to operate.[1][10] The switch failure should have triggered an alarm, but it too appears to have failed.[1][17] Forty-one minutes later, an estimated 300 tonnes of petrol would have spilled down the side of the tank through the roof vents onto the ground inside a bund wall—a semi-enclosed compound surrounding several tanks.[1][4][17]
An overflow such as this results in the rapid formation of a rich fuel and air vapour. CCTV footage showed such a vapour flowing out the bund wall from around 05:38. By 05:50 the vapour started flowing off the site, near the junction of Cherry Tree and Buncefield Lane.[10] Around 05:50 the rate at which fuel was being pumped into the tank increased dramatically. Initially the fuel was pumped in at 550 cubic metres (19,423 cu ft) per hour, but it increased to about 890 cubic metres (31,430 cu ft) per hour.[10] By 06:01, when the first explosion occurred, the cloud which was initially about 1 metre (3 ft) deep, thickened to 2 metres (7 ft) and had spread beyond the boundaries of the site.