Nearly a third of Britain's nuclear power reactors have been out of action due to breakdowns and maintenance.
Hartlepool has seen both reactors closed down
Of the UK's 19 reactors, a total of seven have been shut down.
British Energy, which operates the affected sites, said a "conservative decision" had been taken to close units after safety checks uncovered faults.
Former environment minister Michael Meacher said it was "extremely worrying that one of the major sources of electricity is half-down at this time".
Torness, Dungeness B, Hartlepool and Heysham - all of which comprise two reactors - had been scheduled for routine repair and maintenance shut-downs at one or both of their units.
But after problems emerged at the boiler closure units at Hartlepool Reactor 2, British Energy decided to close two further units at Hartlepool and Heysham.
Torness: both units closed for repairs to electricity supplies
Dungeness B: one of two units out for standard refuelling
Hartlepool: Reactor 1 out for statutory maintenance, Reactor 2 closed following problems with boiler closure units
Heysham: one unit down for refuelling, a second closed for safety checks
And the return to service of the reactor which was undergoing maintenance in Heysham will be delayed.
Mr Meacher, an opponent of nuclear power, said: "I think it is pretty worrying that they themselves have a major problem as a result of breakdown and the need for repair and maintenance.
"One needs certainty, and the nuclear industry doesn't provide it."
Nuclear expert Prof Ian Fells, an advisor to the World Energy Council, told the BBC's Today programme that he blamed a lack of investment for the outages, and warned that the consequences could be serious.
He said: "It is disturbing. I mean, we're going to have to rely on it being a warm winter.
"If it isn't, maybe we will run into power cuts. And the consequential loss of that is not just the loss of electricity, it's the loss to industry when everything fails.
"It's a really quite serious matter."