Security practices at Europe's biggest hospital have been branded "lax" after it was found that mothers are at risk of leaving "with the wrong baby".
Some babies at The Royal London Hospital were found without name tags, meaning they could accidentally be mixed up and even given medication meant for another baby, the Care Quality Commission (CQC) said.
Inspectors said there was also a "lax" practice of checking babies' name bands.
Their report said it could create a "risk that a baby might receive medication intended for another baby, and mother might leave the unit with the wrong baby."
Meanwhile another hospital run by the same organisation, Whipps Cross University Hospital, was lambasted by inspectors who said they did not find a single element of outstanding care across the whole hospital.
Inspectors observed one dying patient at the East London hospital asking for vegetarian food only to be told by a nurse: "You will not get vegetarian diet in here. Where you think you are, in a hotel?"
Other patients nearing the end of their lives were left to suffer in pain.
At The Royal London, inspectors also found there were not enough midwives on the delivery suite to provide safe cover for all women.
And midwives said they had been ordered by managers not to raise concerns about low staff numbers.
There was a "mixed" view about how caring staff were at the hospital - one mother told inspectors she was treated as "childish" because she was upset that her baby had been taken into special care.
Inspectors who visited The Royal London in July this year said they also observed some "intra-cultural issues and some bullying behaviour" both between groups of midwives and between midwives and patients.
Doctors and midwives on the postnatal ward referred to patients by their bed numbers rather than by name, the CQC said.
Last year 4,645 babies were born at The Royal London - the largest standalone acute hospital building in Europe.
The CQC has ordered the Barts Health NHS Trust, which runs the East London hospitals, to "urgently" improve security in maternity after rating the service "inadequate".
The regulator has also said Whipps Cross must improve on a number of key areas - including patient pain management.
On The Royal London, Professor Sir Mike Richards, chief inspector of hospitals at the CQC, said: "We were most concerned about the standard of care around maternity and gynaecology services.
"Staffing on maternity wards was sometimes inadequately covered - but most worrying of all was the lack of a safe and secure environment for new born babies.
"At the time of our inspection, we raised this with the Royal London Hospital as a matter for their urgent attention."
A spokeswoman for the Trust said: "We acted immediately to improve the security of babies at The Royal London Hospital.
"It's important to stress that these reports are based on observations from five months ago - since then we have subjected our processes and procedures to forensic scrutiny.
"As a result we've introduced new baby ID tags, we've reviewed our procedure for locking down the hospital and refreshed our policy.
"We have recirculated our policy to all staff and now test it every single month.
"Women should be assured our services are safe and we will review our processes regularly to ensure they remain safe."
The inspection report for Whipps Cross highlighted a number of areas of concern - particularly surrounding dying patients.
Inspectors said: "On a few instances, during our unannounced visit, we observed patients were visibly in pain, but staff did not respond to this by providing them with analgesia."
The CQC said there were "many examples of a lack of compassion towards patients nearing the end of their lives".
"One patient looked dirty with stains all down the front of their nightwear and staff had neither noticed it nor took any actions to wash and care for the patient," the report states.
The regulator also raised concerns about infection control practice in the surgery service - having observed some staff wearing their surgical scrubs in other parts of the hospital, including the canteen.
The spokeswoman for Barts Health added: "The CQC found that since its last inspection end of life care at Whipps Cross is now safer, more effective and better led.
"At daily safety huddles team will discuss patients who are at the end of their life to ensure we appropriately identify and provide them with safe and compassionate care.
"We have also made a quiet space for sensitive conversations to be held in privacy, and overhauled our training in end of life care with the oversight of Board-level leadership to ensure progress."
On Whipps Cross, Sir Mike said: "Overall, we have rated the service provided by Whipps Cross as Inadequate.
"In the past year there have been some big changes in the management at Whipps Cross and the hospital is moving in the right direction."