Police have interviewed four women under caution in a criminal investigation into the treatment of about 30 elderly people at a London care home, the management of which was outsourced to a private company by a flagship Conservative council in a bid to save money.

Care regulators rated Garside House nursing home as inadequate last month following inspections prompted by a whistleblower. The Care Quality Commission discovered dehydration, serious medication errors, wrongful deprivation of liberty and staffing levels cut below the level stipulated in the contract with Westminster city council.

The council outsourced the running of the the home to Sanctuary Care in 2015 as part of an eight-year contract worth £126m to “deliver savings”. Sanctuary made £77m in profit last year but paid some staff £8.54 an hour, £2 less than the London living wage, a voluntary minimum that Westminster pays its own workers.

Among the problems discovered were out-of-reach bell-cords that left residents with complex needs unable to call for help, staff failing to answer when residents pressed their wrist alarms, medicines running out and people not being given enough to drink that led to one person being admitted to hospital.

“People were placed at undue risk of harm,” the CQC said. “We have identified breaches of regulation in relation to safe care and treatment, safeguarding people from abuse, person centred care, dignity and respect, nutrition and hydration, staffing, acting on complaints and good governance.”

Staff were not properly trained in moving people, end of life care and fire safety, the regulator found. Sixteen people had been assessed as incapable of making decisions for themselves, but the legal process had not been followed and no applications made to deprive people of their liberty. One person’s care plan said they were incapable of communicating their preferences when it was clear they were capable.

When inspectors visited they heard one person shouting in distress while being looked after. Bedroom walls were stained, one person had not had a duvet cover for four days and some people had no toothpaste and shower gel.

A Metropolitan police spokesman said: “An investigation is under way, working with partner agencies including Westminster city council and the Care Quality Commission. Four women have been interviewed under caution. No arrests have been made. Inquiries continue.”

Adam Hug, the leader of the opposition Labour group in Westminster, said: “The CQC’s findings are a damning assessment of the quality of care being provided by Sanctuary that has let down Westminster residents and put them at risk of harm. The council must take action now to finally end the contract with Sanctuary.”

Heather Acton, Westminster’s cabinet member for public health, said: “It’s completely unacceptable that anyone should be treated with anything but the utmost care and respect at a time of life when they need it most. This was obviously not happening and I’m saddened by the fact that some residents have experienced this poor level of care.

“The police are still investigating and we will support them in any way we can. We are carefully considering the next steps with the NHS, but we’re clear that the safety and care of our residents remains our priority.”

Sarah Clarke-Kuehn, the group director for care at Sanctuary Care, said: “We are very sorry that the standard of care at Garside House fell far short of that which we aim to provide. In October 2019, we moved a number of senior managers into the home to oversee an accelerated improvement plan and excellent progress has already been made.”

The CQC inspections took place between 5 and 12 November 2019.

