Headteachers at every school in West Sussex have written to parents warning they may be forced to close for one day a week unless they receive emergency financial assistance from the government.

In a letter, which all primary, secondary and special schools in the county sent out, headteachers warned the “crippling effect” of consistent underfunding has prompted schools to consider “modifying opening hours” in a bid to cut costs.

The move follows repeated complaints that West Sussex has been unfairly underfunded.

The letter says that schools have already made “every conceivable cut to our provision”, and are now faced with taking drastic action.

The county received £44m less than the national average, and £200m less than the average London borough this year, according to the Worth Less? campaign, which is calling for fair funding for schools in West Sussex.

The campaign is urging the government to pledge a minimum of £20m in funding to plug the gap in funds until April when the new financial year begins.

Peter Woodman, head of the Weald school in Billingshurst and chair of the West Sussex Secondary Heads’ executive told the Mail on Sunday: “What is frightening is when you start saying ‘do we have enough money to open five days a week?’

“We're not saying we're going there yet. We're saying on the range of things, absolutely everything needs to be explored.”

He told the Guardian: “But we are considering all the options and that’s one option we have to consider. If we are trying to save money there are limited things we can do.”

Mark Anstiss, headteacher of a school in Felpham said: “It would be terrible. I recognise the burden it would place on parents with the childcare costs and so on that they would have to incur if we did this. But we can’t run the school the way that we have been with the money we get at the moment.”

He added: “I really am scratching my head to think about where we can make more cuts. All the obvious things we can do, we’ve done over the years. It just seems really unfair.”

Schools are also considering cutting class sizes, reducing cleaning and maintenance, buying fewer books and computers, implementing a more basic curriculum, and not replacing staff who leave.

But many schools have already taken these steps and are still looking to save money.

The Department for Education responded to say it was increasing its budget for schools and said it was “firmly committed to introducing a national funding formula so that all schools are funded fairly – both to address the historic unfairness in the system and also so that areas with the highest need attract the most funding.”

“We want all schools, including those in West Sussex, to have access to the resources they need so that every pupil regardless of background or ability can reach their full potential. That’s why we have protected the schools budget so that, as pupil numbers increase, so will the amount of money for our schools – in 2016-17 that will total over £40bn, the highest on record.”

The world's toughest school run Show all 5 1 /5 The world's toughest school run The world's toughest school run Children carry their schoolbags climb on a cliff on their way home in Zhaojue county in southwest China's Sichuan province Chinatopix/AP The world's toughest school run Children carry their schoolbags accompanied by adults climb on a cliff bu using ladder as they on their way home in Zhaojue county in southwest China's Sichuan province Chinatopix/AP The world's toughest school run A child carry her schoolbag looks as she takes a rest on a cliff as she and other children on their way back to home in Zhaojue county in southwest China's Sichuan province Chinatopix/AP The world's toughest school run A village in China's mountainous west where schoolchildren must climb an 800-meter (2,625-foot)-high bamboo ladder secured to a sheer cliff face may get a set of steel stairs to improve safety Chinatopix/AP The world's toughest school run Schoolchildren carry their schoolbags climb on a cliff on their way home in Zhaojue county in southwest China's Sichuan province Chinatopix/AP