Most inpatient care is provided at Basildon University Hospital, which has 777 beds, and outpatient care at Orsett hospital in Grays.
Trust chairman Michael Large said: "It is an extremely serious matter and we do not seek to underestimate its gravity.
"I want to reassure our local community that the safety and well-being of our patients is our highest priority."
The trust said more nurses have now been taken on, and a new emergency department is planned.
The Care Quality Commission (CQC) rated the trust as "good" overall in October.
But a new report from an unannounced inspection team carried out by the CQC found evidence of sub-standard care.
The inspectors saw:
• Floors and curtains stained with blood
• Blood-splattered on trays used to carry equipment
• Badly soiled mattresses in the A&E department with stains soaked through to the foam filling
• Items that should only be used once still in use
• Equipment in the resuscitation room that was past the use-by date
• A children's blood pressure cuff heavily stained with blood
• Suction machines contaminated with fluid inside and out with what looked like mould growing on the equipment
The inspectors criticised a poor care environment in A&E, in particular a lack of privacy for patients.
They also highlighted inadequate arrangements to treat children, with few specialist paediatric staff.
Mother on son's NHS treatment
The CQC has asked the regulator of foundation trusts, Monitor, to take action.
An expert taskforce will be sent into the trust with a remit to drive rapid improvements in patient care.
Monitor chairman Dr William Moyes said: "We will be reviewing the trust's performance regularly and in detail - if we don't see measurable results quickly, we'll take further action."
The trust was one of the first in England to be granted foundation trust status in 2004. That gives the trust greater freedom to manage its finances.
The independent hospital analysis website Dr Foster has found the Essex trust to have a hospital standardised mortality ratio of 136.
This means the rate of death among patients at the trust is a third higher than would be expected by looking at national figures, after adjusting for patients' age and the severity of their illness.
However, senior doctors at the hospital say they have investigated death rates, and do not believe there is a serious problem.
Cynthia Bower, CQC chief executive, said swift action was needed to "nip problems in the bud".
She said: "Our work has uncovered serious failings. The trust has high mortality rates for emergency admissions and we have found evidence of significant problems in different parts of the organisation.
"The trust has taken our concerns seriously but improvements are simply not happening fast enough.
"Our confidence in the management's ability to deliver on commitments and to turn the situation around has been severely dented."
Baroness Young, CQC chair, admitted the current inspection system - which the commission inherited from its predecessor - was flawed, and said reforms would soon be in place.
Health Minister Mike O'Brien said: "Patient safety must be a top priority for the NHS and all patients deserve the highest quality of care."
"We expect these issues to be dealt with quickly and effectively to ensure high quality, safe care for patients."
Mattresses were badly soiled
Andrew Lansley, shadow health secretary, said: "I am extremely disturbed by this news and the effect that these shocking conditions may have had on patients.
"When the appalling standards of care at Stafford Hospital were revealed we were assured by Labour Ministers that it was "an isolated case" - that sort of complacency is simply not good enough."
Norman Lamb, Liberal Democrats health spokesman, said: "If these reports are accurate then it appears there has been a shocking failure in the standards of care at this hospital.
"People have a right to know how on earth a hospital can be rated 'good' a few weeks before such serious failings come to light."
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