An investigation has been launched into a "bankrupt" NHS Trust as concerns grow over the avoidable deaths of mothers in labour, delays in treatment, equipment shortages and poor training.

Inspectors from the Care Quality Commission, the health and social care watchdog, are to go into Barking Havering and Redbridge University Hospitals NHS Trust in Essex after mounting pressure from MPs and patients.

The wide-ranging investigation, only the second of its kind, comes despite repeated earlier assurances from the health trust and the watchdog that improvements had already been made.

The CQC published a damning report into maternity services at Queen's Hospital in Romford in March. But women have since continued to report poor pain control, rude and unsympathetic midwives, and long periods in which they have been left unattended during labour. Five women have died at the hospital in the last 18 months, including Serena Ali who died in January along with her new-born baby.

The inquiry precedes a report next month into the trust's two A&E departments, which is expected to reveal delays in emergency surgery. MPs and safety campaigners welcomed the announcement last night but questioned the regulator's "surprising" decision to lift seven of the eight improvement conditions it imposed on the trust in April 2010.

Margaret Hodge, the MP for Barking, who called for an independent investigation during a parliamentary debate last week, said she had continued to hear "horror stories" from patients since the watchdog had lifted the conditions. Ms Hodge said: "The CQC must get under the spiel of the hospital administrators and really talk to the patients, because these are not people who normally complain.

"The hospital trust is basically bankrupt, and NHS London must clear its huge debt of around £135m or things will never improve. It would be deeply unfair to local people if this hospital is allowed to fail under the new reforms."

Peter Walsh, chief executive of the charity Action against Medical Accidents, said: "We are relieved that the CQC is to conduct a full investigation. However, it should not take several deaths, a parliamentary debate and the glare of publicity before the regulator steps in. We are surprised that nearly all the previous conditions were lifted when clearly there are ongoing problems."

Sarah Harman, from Harman & Harman Solicitors, said some of the deaths might have been avoided had the trust dealt properly with complaints dating back to February 2009. The firm is dealing with 40 cases relating to alleged sub-standard maternity care at Queen's, some from the last three months. The trust has payed out £15m for obstetrics-related claims in the last five years.

Ms Harman said: "Serena Ali died in intensive care while CQC inspectors were inspecting a maternity ward, and yet she fell under their radar... We are concerned that this will turn out to be another tick-box exercise."

The trust's chief executive, Averil Dongworth, said: "The trust has been working hard to drive forward changes. The CQC recognises that some improvements have already been made, although we need to go further."

Colin Hough, CQC's director for London, said: "We've been monitoring this trust very closely, making frequent unannounced visits, talking to patients and staff – and we keep finding problems. Some progress has been made, but not enough."

A Department of Health spokeswoman said that "sub-standard care will not be tolerated" but refused to comment on whether the investigation should have been launched sooner.

Case study

Twenty-nine-year-old Anne-Marie Pitton, a barrister from Gants Hills in Essex, describes giving birth at Queen's Hospital in Romford in April this year as a "terrifying experience".

"I only agreed to have my baby at Queen's after I was assured that things had improved after Serena Ali and her baby died in January. I feel so stupid and angry for believing them. I was left unattended, in agonising pain, for nearly two hours, even though I was a high-risk pregnancy. The midwife was rude, showed me no compassion and made sarcastic comments.

"Me and my baby are fine but it was a terrifying experience because no one was there when I was vulnerable. Things were supposed to be better after the first investigation in March, so I can't help but feel pessimistic about this one."