Ofsted's latest monitoring visit of Tower Hamlets children's services, which it rated as "inadequate" last year, found that in June 160 children did not have an updated assessment - representing more than half the authority's care population.
A letter sent by the inspectorate to the authority said: "Assessments for children in care are not updated routinely, and too often, when cases are in court, the assessments focus on concerns about the parents."
While inspectors agreed the borough had made improvements since a previous visit in May, it warned services still demonstrated "considerable weaknesses" in assessments for children who cannot live with their parents and in permanence planning.
Ofsted handed Tower Hamlets Council's children services its lowest rating in April 2017 after inspectors found it was leaving children in abusive situations for too long and leaders were failing to improve support for children.
The latest visit found the council had made no progress in completing initial health assessments within set timescales when children came into care. Inspectors found it had concluded only 22 per cent of such screenings on time - the same rate as in 2017.
"Many of these children have suffered abuse and neglect," the report warned.
"This means that their immediate health needs are not understood quickly enough."
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The inspectorate also criticised the council's "weak" permanence planning process, which it said was leading to delays for children.
"Some staff described it [permanence planning] to inspectors as ‘ad hoc and fragmented'," it said.
"Options for permanence are not considered simultaneously, which prolongs uncertainty for children. Contingency planning is poor; this is building in delay for some children, including very young children."
In particular, inspectors highlighted that council workers were too slow to consider adoption alongside other options as part of legal proceedings for babies, when parents were unable to care for their children.
"Senior leaders acknowledge that more work is required to change the culture, as adoption or long-term foster care outside of the family are only considered when other options have been exhausted," it said.
Ofsted did find improvements in the authority's access to legal advice, in particular well-attended weekly legal planning meetings, which were helping to prevent delays for children requiring statutory interventions through courts.
It also noted improved performance management arrangements that meant senior leaders and frontline line managers were "very knowledgeable about service performance".
"Routine reporting, disseminated effectively to staff and elected members, is augmented by monthly practice clinics which hold managers to account," said the report.
"Case file audits are completed regularly but more work is needed to sustain the focus on the quality of practice and not just on the process."
Inspectors also found improvements in short-term placement stability, with the proportion of children having three or more placements falling from 14.2 per cent in January 2018 to 11.5 per cent by August.
Tower Hamlets mayor John Biggs said the council was improving its services at "a rapid rate" but that it needed to do more.
"This visit demonstrates continued progress. I am pleased with our overall improvement, but am clear that more work is needed in some areas and at a faster rate," he said.
"We are committed to making our children's services the best they can possibly be and this inspection is the latest milestone on that journey."
The authority's lead member for children, schools and young people Danny Hassell said the council had agreed an action plan and would "work hard to accelerate that pace of change for our looked-after children and ensure a relentless focus on improving in the areas that Ofsted have flagged to us".