Tara Donnelly quit after problems admitting patients in mid-January
The chief executives of two of London's NHS trusts are to leave their jobs, one of them in response to "unacceptable" delays in admitting patients.
Tara Donnelly quit the West Middlesex Hospital in Hounslow after eight new admissions were left without beds for longer than the 12-hour target.
The trust said it also failed to properly handle last month's crisis.
Separately Julian Nettel has stepped down from Barts and the Royal London for "personal reasons".
His trust had also experienced problems with admissions, with 573 patients waiting longer than the six-month target period for treatment between April 2006 and January 2008.
However, a spokesman stressed this was fewer than 1% of all patients seen during this 21-month period and said a mix-up with a computer system had led to the longer waits.
The problems involving eight patients at the West Middlesex occurred on 19 January.
This was when the hospital "faced heavy demand for its services due to winter pressures", trust chair Sue Ellen said.
"Tara, as leader of the organisation, recognised the seriousness of this and therefore decided that the most appropriate course was for her to step down," Ms Ellen added.
She said the trust's board had started an investigation to keep waiting times under control in the future, while separately an independent team will review the trust's emergency admissions procedures.
Barts and the Royal London NHS trust comprises St Bartholomew's Hospital - known as Barts - in the City, the Royal London Hospital in Whitechapel and the London Chest Hospital in Bethnal Green.
A spokesman said the trust had "some of the finest clinical services in the country", but acknowledged there were "some weaknesses in our management and administration systems".
"These have resulted in some patients waiting longer than they should have done.
"The trust apologised directly to the patients affected and reported the issues to NHS London as soon as they came to light."
'Too few beds'
Geoff Martin from pressure group London Health Emergency said the two resignations had "plunged London's NHS into chaos" and left behind a "mess".
Patients were being put at risk because there were too few emergency beds, he added.
And he called for the government to stage a full public inquiry into London's emergency health provision.
But the Barts and Royal London spokesman responded by saying: "You cannot compare the situation at West Middlesex Hospital with Julian Nettel's personal reasons for stepping down at Barts and the London NHS Trust.
"There is no comparison to be drawn between the two."