The Care Quality Commission has rated the Tower Bridge Care Home at Bricklayers Arms as 'inadequate' and has placed the home under special measures following an inspection last month.
The care home is owned by HC-One who took over the centre from Southern Cross in 2011.
At the time of last month's inspection 90 people were living in the building which has capacity for 128. The centre specialises in care for older people with dementia.
During the unannounced inspection, CQC found that the centre was failing to provide care which was safe, effective, caring, well-led and responsive to people's needs.
CQC found that resident's nutrition and treatment needs were not being met, with many enduring long periods of time without a drink or a meal, and some not being provided with the individual care and treatment as advised by healthcare professionals.
Inspectors found that incorrect stock levels of medicines were being kept and residents on occasion did not always receive their medicines as prescribed.
"It is essential that the Tower Bridge Care Centre takes action to address the concerns we identified during the last inspection," said Sally Warren, CQC's deputy chief inspector of adult social care.
"The provider has a responsibility to ensure that people living at the home receive care that is responsive to their needs and protection from the risk of harm at all times.
"When we are faced with a residential or nursing home that is failing to care for residents, our first instinct is to work with the provider to ensure that the service improves.
"We will re-inspect the home within six months to check whether sufficient improvements have been made.
"If sufficient improvements have not been made and there remains a rating of inadequate for any key question or overall, we will take further action which may include cancelling the registration of the service."
An HC-One spokesman said: "Nothing is more important to us than the health, safety and wellbeing of each person we support.
"We took the points raised by the Care Quality Commission extremely seriously, and immediately implemented a comprehensive and robust action plan to address their feedback.
"Since the inspection we have implemented a range of measures including: appointing an experienced new home manager and senior clinical lead; recruiting additional unit managers and nurses; improved our medicines management processes; conducted a personalised review of all resident care; and implementing a robust medicines management action plan.
"A dedicated training plan is also in place, with a new learning coordinator appointed, regular reviews from our regional learning and development team, and all staff receiving additional training.
"We continue to work extremely hard to ensure we're providing the kind care residents expect and deserve. "