EastEnders' ratings have fallen to fewer than five million viewers.
The programme, which recently lost out to Coronation Street for Best Soap at the British Soap Awards for the second year running, drew just 4.8 million viewers on Tuesday.
In 2010, it notched up an audience of 16.6 viewers when Archie Mitchell's killer was revealed live on the show's 25th anniversary episode.
The slump has put it below Emmerdale's 5.1 million viewers, which it was suddenly up against when broadcast was delayed to allow coverage of Kate Middleton and Prince William introducing Prince George to the world outside St Mary's Hospital in London.
One show insider told the Sun: "All the soaps are struggling at the moment."
The ratings woes follow announcements that a number of well-known past favourites would be making dramatic comebacks.
Her return to Albert Square will coincide with Samantha Womack reprising her role as baby-snatcher Ronnie Mitchell and Michael French returning as womaniser David Wicks.
Executive are also planning a spectacular £1m car crash which will leave Phil Mitchell fighting for his life.
"We got seven million on Monday. We're not worried - there are some huge storylines planned for the autumn," said the source.
"Good storylines take months to plan. We're fully expecting the soap to recover. There are always peaks and troughs."