United Kingdom — The fires broke out at the southern end of Epping Forest, according to reports.
The first fire saw an area of land the size of nine football pitches alight but is now believed to be under control.
A second, smaller fire, is still being put out by firefighters.
Station Manager James Morford said: Our crews have been working incredibly hard to bring these two fires under control in what are extremely hot and arduous conditions.
We want people to enjoy the sun this weekend, but we would also urge people to take a few simple precautions to help us prevent grass fires like this from starting.
A small spark from a cigarette is often all it takes to start a grass fire in these dry conditions. It really is important that smokers dispose of their cigarettes properly. Drivers also need to take care not to throw cigarettes out of car windows as they can easily burn grass verges.
If you see a grass fire, dont attempt to put it out yourself as grass fires can be fast moving and change direction without warning. Call the fire brigade.
Wildfires could spread throughout Britain as the country sweats in the longest heatwave for seven years, it is feared.
There is an increased risk of blazes in the countryside, with rainfall at just 15 per cent of the average monthly total so far, the Met Office warned.
The past six consecutive days of above 30C (86F) temperatures could also cause up to 760 people to die prematurely, it was claimed.
I can confirm that the advice given to our governmental partners is that there is an elevated risk of fires in the next couple of days, a Met Office spokesman said.
A blaze also burned through grass and gorse the equivalent size of four football pitches in Mitcham, south London, before it was brought under control.
London Fire Brigade has dealt with twice as many grass fires in this summers heatwave compared with last year as Britain baked in the hottest day of the year on Thursday, with the mercury rising to 32.2C (90F).
Heatwave health warnings in the south-west of England and the West Midlands were increased from level two to three by the Met Office, putting the regions on a par with the south east and London.
The East Midlands, Yorkshire and the Humber and the east remain at level two. The north east and north west are cooler at level one.
Health officials repeated advice to beat the heat by drinking lots of cold fluids and urged people to look after those who may suffer more in the hot weather.
Dr Angie Bone, heatwave plan leader for Public Health England, said: In this continued hot weather, its important to remember that high temperatures can be dangerous, especially for people who may be vulnerable such as older people, young children and those with serious illnesses.
During very hot weather, pregnant women and people who have chronic illnesses, including cardiovascular, respiratory, renal conditions, diabetes or Parkinsons disease, may experience discomfort if indoor temperatures are particularly hot and in using public transport.
A four-week-old baby was among ten children admitted to one hospital in the south east with sunburn since the sunny spell started.
Six of the youngsters needed specialist treatment after being referred to the unit from other hospitals because of the severity of their burns.
In Wales, doctors at the Abertawe Bro Morgannwg University Health Board warned of the dangers of ride-on lawnmowers as surgery trauma cases doubled to 53.
Dean Boyce, consultant plastic surgeon at the Welsh Centre for Burns and Plastic Surgery at Morriston Hospital, in Swansea, said: Unfortunately we are treating a patient with a very severe foot injury, and this is not the first time this has happened.
We have seen children who have lost a foot after falling off a parents lap riding a ride-on mower.
We would warn people to avoid letting anyone ride on their lap for fun because in fact this is very dangerous, as the mowers safety cut out will not engage if they fall off.
And there is little sign that the summer is set to cool, according to MeteoGroup.
Tomorrow the West Midlands, central southern England and the West Country could see maximums of 28C (82.4) to 30C (86F), said forecaster Gemma Plumb.
On Saturday and Sunday it looks like we will have temperatures of around 28C but by Monday we have temperatures of 29C and 30C again, with an increased risk of getting some showers.
There is a likelihood of thunderstorms from Sunday and into Monday, initially in the west of England and South Wales and then heading into the south of England, but these would not necessarily bring temperatures down.
Police also warned people not to leave dogs in cars even with windows open.
And authorities repeated concerns about people trying to escape the heat by swimming in open water after a number of deaths in recent weeks.
Four people died in separate incidents on Tuesday in lakes, rivers and the sea in Norfolk, the Shropshire-north Wales border and Cornwall.