Furious last minute lobbying for additional funding for schools, colleges and pupils with special educational needs and disabilities (SEND) is under way in the run-up to next week’s spending review.

Leaders of the biggest teaching unions had an hour-long meeting with the education secretary, Gavin Williamson, on Thursday, and made their case for more money for struggling schools as part of a long-term funding plan.

Letters have also been sent to the Department for Education (DfE) and the Treasury, urging the government to boost funding for special needs and post-16 education, which have been hard hit in recent years.

Earlier this week the Guardian revealed confidential plans by the government for a raft of education measures including billions of pounds in funding, a crackdown on student behaviour and a further wave of free schools, due to be announced in time for the start of the new school year.

Williamson told union leaders he would not comment on the leaked document, which included plans for an extra £2.8bn for primary and secondary schools up to the age of 16, including £800m for children with SEND. Details of an extra £800m for sixth form and further education colleges were said to be still under discussion with the Treasury.

Williamson said: “I’m looking forward to working with all our education unions on our shared goal of giving children and young people of all ages the best possible education and laying the foundations for success in whatever path they choose.”

Pupils with special educational needs to stage funding protests Read more

A cross-party group of 93 MPs has signed a letter to the chancellor, Sajid Javid, urging him to prioritise education funding for 16- to 18-year-olds. Signatories include Robert Halfon, the chair of the education select committee, and several former Conservative ministers.

The letter calls on the chancellor to raise the base rate of funding from £4,000 to at least £4,760 per student, rising in line with inflation. “It is extraordinary that 16-to-18 education has for so long been starved of cash,” said Halfon. “Funding this age group properly must sit at the heart of a 10-year plan and be a major priority in the forthcoming spending round.”

There have been calls that funding for education should be looked at in a similar way to the NHS but according to those at the meeting, a 10-year plan may not be in Williamson’s gift. Geoff Barton, the general secretary of the Association of School and College Leaders, said: “Warm words and lingering promises simply won’t do.

“We said to the secretary of state for education at our meeting today what we’ve been saying in public for months – that there needs to be an urgent response to the current funding shortfall, plus a longer uplift that builds a sustainable education system for all children and young people, irrespective of their backgrounds.”

Another letter sent to the DfE highlighted the crisis in SEND funding in the capital. Signed by 20 boroughs, it said there was a £77m shortfall in funding for SEND services across London last year.

Danny Hassell, the lead signatory and Tower Hamlets council cabinet member for children, schools and young people, said: “With demand for services rapidly increasing but funding at a standstill, we have now reached a tipping point with potentially very serious consequences.”

A DfE spokesperson said spending on children and young people with the most complex SEND has increased from £5bn in 2013 to over £6bn this year. “The prime minister has also made clear that we will increase minimum levels of per pupil funding in primary and secondary schools. Further details will be announced in due course.”

The Treasury is being urged to prioritise social mobility in the spending review. The all-party parliamentary group on social mobility has written to the chancellor backing calls for more investment in further education to help boost low levels of social mobility across the country.