But cash-strapped head teachers say the money will have to come out of budgets earmarked for children's education.

One angry head accused Royal Mail of breaching its' contract for failing to deliver post already paid for.

Under new rules schools will have to pay for mail that cannot be handed over to staff on site either at weekends or during the 13 weeks' holiday.

Former Preston headteacher and Lancashire National Association of Headteachers' spokesman said: "It is ridiculous. What they are saying is that if schools don't have a caretaker on site, which many of them don't these days, the mail won't be delivered and schools will have to pay to get it back when they re-open.

"The problem is that in urban areas schools don't have letterboxes so mail has to be officially handed over.

"The problem is compounded by the fact that so many schools have removed their letter boxes as they are a target for vandals."

Mr Roberts said some schools were refusing to pay and others had complained that valuable mail, including pay slips, invoices and references were being mislaid after being returned to sorting offices.

It is believed small schools could face collection bills of around £200 – the daily cost of a supply teacher.

Brendan Hassett, headteacher at Dolphinholme CE School near Lancaster added some Lancashire schools allowed their premises to be used as bases for mail to be stored there for a number of delivery routes.

Mr Hassett said: " We have a member of staff living next door so it is not a problem for me but I think it is wrong to charge for a service they are not providing."

He added "My concern is that people have paid for stamps for their mail to be delivered and if it is not being delivered why should we be charged for them not doing their job?

"The kind of money they are talking about would pay for a supply teacher or my paper and pencil bill for a term."

Martin Jelley, head at Penwortham Primary School said: "It hasn't affected me yet but the postmen use this as a base to drop their base off so they don't have to carry it around."

A Royal Mail spokesman said European Competition Law requires that all businesses such as Royal Mail must ensure that a level playing field is provided for all customers.

He added: "To make an exception, would leave us open to potential litigation for failing to treat all our customers in a consistent manner.

"Furthermore, the opening of the postal market has meant that we must not be seen to be acting in a manner that is in any way anti-competitive or we could face fines of up to 10% of our annual turnover.

"As such, any organisation that is not a social customer must be treated in the same way, regardless of whether they area business, charity or school."

Colleagues beware, says a worried head

One letter from a worried head reads:

We first had a problem with this over Easter when Royal mail didn't keep our mail – but failed to inform us of the change to their operation.
Very annoying,but nobody would accept responsibility
or apologise so did have a problem not knowing what was missing,what had been returned etc.
At half-term we paid £28 for them to keep our mail-which if I understand it is the standard charge,but still lost mail because we did not give them time to check our application details-would have thought a week was long enough! Colleagues beware.