A newly-opened academy is set to lose out the most in the government’s National Funding Formula, according to predicted data.

Described as a “historic reform” by former education secretary Justine Greening, the formula, which comes into effect this month, sees an increase in spending on schools nationally by £1.3billion over the next two years.

But education unions have been at pains to point out that the majority of schools are still running at a deficit longer term.

According to their figures – based on government data and Institute for Fiscal Studies calculations – Islington schools will have lost £6.4million between 2015 and 2020.

This is the equivalent of losing £309 per student over the same period.

Archway’s City of London Academy Highgate Hill, formerly Mount Carmel Catholic Girls College, is predicted to experience the biggest funding loss in that period between 2015 and 2020.

The school, which opened in September, has been earmarked to lose almost £1.5m – the equivalent of £4,129 per pupil – over the stretch, due to the sharp increase in class sizes.

Also set to feel the pinch over the same period are Central Foundation Boys’ School (-£427,218), St Mary Magdalene Academy (-£302,732) and City of London Academy Islington (-£253,868).

Paul Whiteman, general secretary of school leaders’ union of the National Assocation of Head Teachers (NAHT), said: “We desperately need the government to recognise that overall funding for schools is insufficient.

“It is totally unrealistic to ask school leaders to plan efficiently and effectively for the long term if they have no information about what will happen in as little as two years’ time.

“All schools need significant increases in funding. And as they fail to materialise, some school leaders could believe that the formula is the problem.”

The two key factors behind the union’s calculations are government cash being reallocated across the country and funding not rising with inflation.

According to the data, the hardest hit primary school over the same period is Ambler Primary School and Children’s Centre (-£199,762).

To check how your school will be affected by budget cuts, visit schoolcuts.org.uk