The Met Office warned that the capital would bear the brunt of the heat with possibly the hottest day of the year so far.
Porthmadog in north Wales reached 91F on June 28.
Met Office spokeswoman Nicola Maxey said: “We could well see the hottest day of the year so far this weekend. It’s only got to go to 33.1C and you’re there.”
And as forecasters said no rain is in sight, water companies were last night desperately urging people to use water wisely or face a formal hosepipe ban as early as next week.
United Utilities, which supplies water to the north-west of England, warned it was experiencing record levels of demand.
A spokeswoman said: “If demand doesn’t reduce we will have to seriously consider introducing a formal hosepipe ban.”
She warned last night that the ban could come into place “within days, rather than weeks”. Restrictions have already been imposed by Northern Ireland Water. Elsewhere across the UK, consumers are being urged to reduce the amount of “precious” water they use.
Utility companies are urging Britons to shower for only four minutes, let their lawns turn brown and wait for the heatwave to end before they wash their cars.
Industry body Water UK warned water companies are seeing “extremely high levels of demand” with billions more litres of water being supplied across the UK.
And Thames Water has ramped up its supply to meet soaring demand.
Severn Trent Water has asked people to be “careful with their water” and avoid using the garden sprinkler or hosepipe.
The threat comes as forecasters say Britain is set to bake in a heatwave for the rest of the month as the fine weather shows no sign of ending.
Met Office forecaster Simon Partridge said last night: “There’s no let-up in sight – it’s pretty much dry across the board and there is no sign of rain.”
Mr Partridge said the UK faces weeks of higher-than-average temperatures in the longest heatwave in over 40 years, but that temperatures will fall slightly to the mid-70Fs next week.
The scorching temperatures even saw an unfortunate koi carp suffer sunburn. The fish was rushed to vets in Burton-on-Trent this week after its scales were scorched, but has now fully recovered.
Scotland might see some rain in the coming days, said Met Office forecaster Emma Salter, but the rest of the country could expect it to stay dry until the middle of July.
Water companies across the UK were last night “monitoring the situation closely”, while Ofwat said it was watching how companies were responding to the heatwave.
United Utilities said: “We urgently need your help and support to avoid a hosepipe ban. You can help us by turning off your garden sprinklers, not using your hosepipes, not washing your car and taking shorter showers.”
The company issued a statement last night, saying: “The prolonged hot weather means the region’s reservoir stores have decreased since last week, with some reservoirs looking more dramatic than others.
“We are still encouraging our customers to voluntarily use water efficiently…however, if demand does not reduce in the next few days we’ll have to introduce enforced restrictions.”
Water UK said Britain was not facing a “drought” due to “above average rainfall” this spring.