There was an alarming 20% rise in babies being killed or harmed during the first lockdown, Ofsted's chief inspector Amanda Spielman has revealed.
Sixty four babies were deliberately harmed in England - eight of whom died. Some 40% of the 300 incidents reported involved infants, up a fifth on 2019.
Ms Spielman believes a "toxic mix" of isolation, poverty and mental illness caused the March to October spike.
Health staff and social workers were hampered by Covid restrictions.
And many regular visits could not take place, while others were carried out remotely, using the telephone or video links.
When schools were closed in March, children's charities and teachers expressed fears that children at risk would be left even more vulnerable under lockdown. This was part of the reason schools stayed open to vulnerable children.
Ms Spielman told a conference of local authority leaders on Friday: "Of course, babies can't tell an adult if there's a problem.
"Often, abuse is only uncovered when there's a critical injury, or it's too late.
"Another young life damaged, and in the worst cases, lost, before it's really had the chance to begin… It doesn't bear thinking about.
"But we must all be alive to this hidden danger."
Every week, Ofsted is seeing more serious incident notifications about harm to under-ones - the youngest and most vulnerable of all children, she said.
'Covid pressure cooker'
She added: "The pandemic has brought difficult and stressful times. Financial hardship, loss of employment, isolation, and close family proximity have put extra pressure on families that were already struggling.
"Poverty, inadequate housing, substance misuse and poor mental health all add to this toxic mix.
"You'll be well aware of the increase in domestic violence incidents over the summer - just one symptom of the Covid pressure cooker."
Violence towards babies was already a worry before the Covid-19 pandemic, Ofsted says.
Over a quarter of all incidents reported to the child safeguarding practice review panel last year involved non-accidental injuries to babies.
This often involves children being abused by young parents, or other family or household members, who have very little social support, Ofsted says.
President of the Association of Directors of Children's Services Jenny Coles said Covid-19 was exacerbating many of the difficulties that families face and putting more vulnerable babies at even more risk.
"The pandemic has seriously disrupted a key line of sight into the lives and homes of many families."
She welcomed the government's move to ensure health visitors were not drawn away from their work with young children, but called for more investment to in services supporting parents.
Accidental baby deaths
As well as babies being intentionally harmed, Ofsted has seen a high number of unexpected infant deaths.
This includes preventable tragedies: babies not being put down to sleep safely, for example, sharing a bed or sofa with a parent who has been drinking.
Ms Spielman will acknowledge that there has been good work to identify high risk babies during lockdown, such as the children of parents misusing substances, or with serious mental health problems.
In these cases, professionals who understand the risk factors acted decisively to give families the help they need, she said.
But she is urging all those working with children to be on the alert as England heads into the next lockdown.
"Everyone needs to play their role. That includes professionals across whole communities.
"Midwives, health visitors, GPs and other health workers who have good relationships with families and can build on that trust. Staff working in schools and nurseries who may have information about a vulnerable infant because an older sibling attends the setting.
"And help for younger parents is important, but without labelling or patronising them. Building their trust so that they accept advice and learn how to look after their children safely.
"Continuing restrictions may be hampering face-to-face visits but while these children are out of sight, they should never be out of mind," she adds.
Children's minister Vicky Ford said: "Every death or harm is tragic but harms to young babies are deeply heart wrenching.
"New parents have faced unprecedented challenges so we have asked children's services to prioritise support for families with new babies and especially those families who had already faced difficulties before Covid."
A spokesman added the government had invested £4.3bn in councils to meet the additional demands being made of them.
Councillor Judith Blake, chairwoman of the Local Government Association's Children and Young People Board, said the pandemic had led to an increasing number of families facing exceptionally difficult circumstances.
"Councils have worked tirelessly to try and keep children and their families safe and well, through online and virtual contact and resources, as well as high priority home visits.
"It is absolutely critical that professionals are able to keep engaging with families throughout any restrictions, whether local or national.
"As we experience a second wave of the virus and learn lessons from the first, it is vital that health visitors and other community health practitioners are not diverted to acute care."