Audience figures for EastEnders dipped sharply on Tuesday as the soap failed to counter competition from an hour-long episode of Emmerdale.
Viewers have said EastEnders needs stronger plotlines
Unofficial overnight ratings showed the BBC One total at only 6.2m compared to 8.1m for ITV1's Emmerdale.
It comes only a day after EastEnders' executive producer Louise Berridge quit over falling ratings.
A spokeswoman for the BBC said: "It's incredibly difficult when the soap opera audience is split."
She said the dip in viewers was unusual as the soap had maintained a steady average over the summer.
On Tuesday, EastEnders attracted only a 30% share of viewers, whereas the ITV soap took 41.5%.
On the previous Tuesday, 9.6 million watched EastEnders, when it was screened against Champions League football on ITV1.
The figures do not take into account viewers who record programmes to watch later.
Ms Berridge left to "move on to new challenges" after ratings fell from an average of 13 million at the start of 2004 to 11 million in June.
In July she defended the soap after a Mirror newspaper survey found that 57% of viewers thought it had lost its appeal.
More than half of those questioned thought EastEnders needed stronger plotlines.
Emmerdale took home three prizes from the British Soap Awards in May, compared to seven for EastEnders.
Kathleen Hutchison, currently executive producer of BBC One hospital series Holby City, is to take over Ms Berridge's role at EastEnders.