The government suspended Ofsted inspections of children's centres in September 2015 "on a short-term basis" pending a consultation on their future, which is still yet to take place.
According to analysis by Action for Children, had inspections not been suspended, an estimated 969 children's centres - around 40 per cent of the total - would have been assessed for quality of performance and impact.
While the suspension has been in place, Ofsted has still been responsible for emergency inspections of centres if a safeguarding concern is raised. However, Action for Children said that a Freedom of Information request it made in July 2017 found that no such emergency inspections were carried out as none were required.
Before the suspension, Ofsted was required to inspect centres no later than five years after the previous inspection. Inspections collect evidence on aspects such as the safety of the centre, how its public finances are managed, how well it serves young children and parents in the area and its success in identifying prospective families in most need.
Chief executive of Action for Children, Sir Tony Hawkhead, said: "The government acknowledges that children's centres have an ‘important role to play' in ensuring all children get the best possible start in life, but the ongoing freeze of Ofsted inspections is undermining them.
"Without evidence from Ofsted about how centres can improve, central government has left local authorities with no clear national standards or framework for these vital services.
"How are we to know how well families are being supported, or the impact of the £1bn of public money spent on children's centres since 2015? Would we allow all schools and hospitals to go uninspected for so long?
"While it continues to drag its feet, central government is letting down tens of thousands of children and families who need support now. It urgently needs to push through its overdue review of early years services so children's centres have clarity about how they should be working, what they should be achieving and the accountability framework they are working to."
The planned consultation on the future of children's centres, and the suspension of Ofsted inspections of them, was announced by former childcare minister Sam Gyimah in July 2015. At the time Gyimah said the government would launch an open consultation on the future of children's centres in autumn 2015.
However, following a series of delays, it was announced in 2016 that the consultation would form part of the government's forthcoming life chances strategy.
In December 2016 it emerged that the life chances strategy had been dropped.
A consultation on the future of children's centres is yet to take place.
Neil Leitch, chief executive of the Pre-school Learning Alliance, said it is "completely unacceptable" that so many children centres have been left uninspected for such a prolonged period of time.
"This was meant to be a temporary pause while ministers consulted on the future on the children's centres - but more than two years later, such plans seem to have disappeared without a trace, leaving those both working in and using such services completely in the dark about their future," he said.
"The government talks a lot about the importance of social mobility - so how can it justify all but abandoning such a vital source of help and support for vulnerable families?
"This has dragged on for long enough. The government must now confirm its plans for children's centres, and ensure that those families that need it most have access to the early support services that they need."