Crews tackling a peatland wildfire at Holme Fen Nature Reserve, near Peterborough, are facing temperatures of up to 60C (140F), according to an area commander. Wayne Swales described the conditions as feeling like "boil in the bag" meals.
The blaze, which broke out on Saturday, is the second at the 660-acre (270-hectare) site in two weeks. Around 50% of the reserve has been burnt.
Mr Swales stated that firefighters are working in "some of the most difficult working conditions," with peat on fire and trees falling. Tactics were changed on Tuesday to reduce resource impact.
The fire service expects crews to remain at the reserve for the rest of the week. The first fire, which started on 27 July, is being investigated as suspected arson by Cambridgeshire Police.
Brigades from across the East of England, including Cambridgeshire, Norfolk, Suffolk, Northamptonshire, Bedfordshire, Hertfordshire, and Essex, have joined forces to combat the flames. Extra crews from Merseyside have been requested to assist.