If ministers are serious about education “catch-up” after the Covid disruption, they should not be considering pushing 800,000 children into “devastating poverty”, the outgoing children’s tsar will say tomorrow.

In a hard-hitting final speech as children’s commissioner, Anne Longfield will say: “Two weeks ago, the prime minister said educational catch-up was the key focus of the entire government - yet we still don’t know if next month he is planning to take the Universal Credit uplift away from millions of families. The two positions aren’t compatible.

“If the government is really focused on educational catch-up, it wouldn’t even countenance pushing 800,000 children into the type of devastating poverty which can have a much bigger impact on their life chances than the school they go to or the catch-up tuition they get.”

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The children’s commissioner will add: “It’s impossible to overstate how damaging the last year has been for many children - particularly those who were already disadvantaged...How many children are in families that are struggling to support them; how many are starting school so far behind they’ll never catch up; how many children with mental health needs or special education needs aren’t getting the help they should be?”

‘A national scandal’: Vulnerable children not getting basic qualifications

Ms Longfield also will call it a “national scandal” that almost a fifth of children “reaches the age of 19 without getting five GCSEs, a technical equivalent or an apprenticeship”.

She will say: “That is abysmal … I don’t know what’s more shocking: that these things happen or that they’re hardly recognised. No one can honestly believe that 20 per cent of children are incapable of achieving basic qualifications. It should be a national scandal.”

Ms Longfield will say that Boris Johnson must show he is serious about vulnerable children by placing them at the heart of his plans to “build back better” post-Covid.

She will warn that the prime minister’s promise to “level up” the country will be “just a slogan” unless children are put “centre stage”. The commissioner will call for a new “Covid Covenant” of education and wellbeing support in every community to help children recover from the pandemic.

A year of opportunity should be launched once the virus is suppressed where schools, sports halls and swimming pools are used at evenings, weekends and holidays to help pupils “catch up with confidence”, she will say.

Ms Longfield will call on Mr Johnson to get “passionate” about making sure that we do not “define children by what’s happened during this year” but instead define ourselves by what we offer them.

“It will take political will and funding - an opportunity fund - measured in billions, but it would be worth every penny. It should be led by the prime minister,” she will say.

Treasury ‘institutionally biased against children’

The children’s commissioner will accuse the Treasury of “institutional bias against children” as only around £1 billion has been committed to pupil catch-up support despite warnings from the Institute for Fiscal Studies (IFS) that school closures could lead to a loss in future earnings of £350 billion for pupils.

Ms Longfield will reflect on her six years as children’s commissioner and talk about her frustration with Whitehall officials failing to tackle many problems facing vulnerable children.

In her speech, she will say: “I have been shocked to discover how many officials have never met any of the children they are responsible for. So many seem to view them as remote concepts or data points on an annual return.

“This is how children fall through the gaps - because too often the people in charge of the systems they need simply don’t see them and try to understand their world.

“I have to force officials and ministers to the table, to watch them sit through a presentation, maybe ask a question, and then vacantly walk away. I do not believe this truly reflects the extent of government and the public’s commitment to helping children succeed,” Ms Longfield will add.

She will conclude: “My parting plea to you is this: please don’t forget about vulnerable children.…These are your children now. You have a chance to put them centre stage. When you do build back better, make sure you do it around them.”

A government spokesperson said: “Protecting vulnerable children has been at the heart of our response to the pandemic, driven by our commitment to level up opportunities and outcomes.

“That’s why we have enabled the most vulnerable children to continue attending school in person, while providing laptops, devices and data packages to those learning at home and ensuring the most disadvantaged children are fed and warm.

“We have also driven forward crucial reform in adoption, in the care system, in post-16 education and in mental health support - and our long-term catch-up plans and investment of over £1 billion will ensure we make up for lost time in education over the course of this Parliament.

“Anne Longfield has been a tireless advocate for children, and we’re grateful for her dedication and her challenge on areas where we can continue raising the bar for the most vulnerable.”

Ms Longfield will step down at the end of the month.