The news, confirmed to CYP Now by Ofsted's director of social care Eleanor Schooling, is the second time that Ofsted has had to extend the deadline for assessing children's social care services at all 152 local authorities using the single inspection framework (SIF).
Launched in November 2013, the SIF was intended to be completed within three years, but last November it emerged that the Department for Education had granted Ofsted an extended deadline of 31 March 2017.
The latest delay, which has been agreed by Ofsted chief inspector Sir Michael Wilshaw, means completion of the programme will be a year later than originally envisaged.
Schooling said: "I think it will take until December [to complete]. The workforce know that and I know that. We may finish earlier, but we need that [time] in order that we get through the programme properly.
"It’s a universal inspection and you can’t say we’ll do some and not others. It will form the baseline for us to go and do our new inspections after the SIF."
By the end of last month, 79 councils had received inspection judgments, while a further 11 have been inspected and are awaiting the outcome.
Children's services leaders have complained that the process is too onerous and diverts resources away from frontline practice.
Schooling said that the intensity of the inspection process had meant that the programme was taking longer to complete than expected.
She added: "It is difficult to schedule – if people are out for a month on an inspection, then they can’t go straight out for another month on an inspection. It does take a bit longer.
"Although the SIF looks massive, and it is, we have got rid of a number of other inspections that were happening on a regular basis. People were always having to be inspection-ready. It is demanding and costly, but the reality."
The SIF has been controversial since its launch, with around three quarters of councils inspected so far being rated in the bottom two categories of "inadequate" or "requires improvement".
But Schooling said the reason for the large number of low judgments was that areas where there were most concerns were inspected first.
"Ofsted prioritised the areas where there was the most anxiety, so you got a skewed picture because you didn’t have a balanced sample of authorities," she explained.
"There will be a more balanced sample of the authorities that remain. I think this will begin to show a different picture as we go forward."