Nearly 70pc of London and the South East’s electricity was being imported from France and the Low Countries on Tuesday. That total reached 74pc on some days last week.
Similarly, about half of north-east England’s electricity was being imported on Tuesday, largely from Norway.
In total, the UK paid £1.7bn for imported power in the first five months of this year. The annual bill is on course to eclipse last year’s total of £3.6bn, of which £1.8bn was paid to France’s state-owned power company EDF.
Its nuclear power stations, especially those on France’s north coast, have become essential to maintaining power supplies for London and the Home Counties.
EDF operates 57 reactors across France. However, overall nuclear output has fallen by more than a tenth so far this year.
‘It is making us poorer’
Supporters of the current UK system argue that backing up the country’s own power grid with imports helps to improve energy security.
Opponents warn that the UK is becoming too reliant on such imports, putting it at the mercy of foreign traders and governments.
Claire Coutinho, the shadow energy secretary, said: “As I have repeatedly warned, the current Government is increasingly putting our energy security in the hands of our Continental neighbours.
“What if politics changes? Do we really want to rely on Marine Le Pen to keep the lights on in Britain?”
Richard Tice, Reform UK’s energy spokesman, said: “We are importing more electricity at this high cost because the growing reliance on intermittent wind and solar power.
“This means higher bills and sending more of our money overseas. It is making us poorer. It also makes us more vulnerable to shortages, rationing and blackouts if there is a problem in Europe. We must reverse course urgently.”
A heatwave in June caused a surge in demand that threatened to overwhelm the grid.
Faced with a disastrous shortage of domestic power, senior engineers from the National Energy System Operator (Neso), UK’s grid operator, had to ring their French counterparts to beg for extra electricity to keep Britain’s lights on.
A Neso spokesman said: “Energy prices are influenced by multiple factors, including weather conditions, and are dictated purely by the wholesale market. Neso has no influence over these prices.”
A spokesman from the Department for Energy Security and Net Zero said the main driver of high energy costs was global gas prices, “which also pushed up electricity prices”.
The spokesman added: “Homegrown power we control is the only way to deliver energy security for the country.”