David Bell said the government had to weigh up its priorities

Chancellor Gordon Brown, in his Budget, said he wanted to raise average investment to private school levels.

But David Bell, permanent secretary at the Department for Education and Skills, said that was an "aspiration". The priorities had to be weighed.

Meanwhile a head teachers' leader is saying a lack of funds could jeopardise government plans for extended schools.

Mr Brown said in March: "We should agree an objective for our country that ... we raise average investment per pupil to today's private school level."

A Conservative member of the Commons education select committee, Rob Wilson, accused Mr Brown on Wednesday of "a piece of headline-grabbing".

Mr Bell - the former chief inspector of England's schools - told the committee no research was being done within his department on the practicalities of meeting the objective.

Asked about his expectations for future funding, he said: "It is going to be tighter, I don't think there is any secret about that. The chancellor has made that very clear."

A spokesman said later the government's long-term aim "remains unchanged".

"We will leave others to distract themselves with debates over semantics."

'Priorities'

Committee chairman Barry Sheerman said the "feeling of euphoria" after Mr Brown's Budget speech had dissipated.

Education has benefited enormously from significant investment over the last nine to 10 years David Bell Department for Education and Skills

The outlook for education was now "rather depressing".

As a spending priority it was slipping into fourth place behind health, criminal justice and transport, he said.

Mr Bell said: "The Treasury, properly, is considering along with departments a whole range of issues in advance of the comprehensive spending review."

Liberal Democrat Stephen Williams said the Institute of Fiscal Studies had predicted that annual increases in education expenditure would fall from the 4.6% it had been since 1997, to 3.4%.

Mr Bell said "the context might be tougher".

"I think we do accept that the context may be different in the future and we will have to drill down hard on priorities."

But spending had soared since Labour had come to power.

"Education has benefited enormously from significant investment over the last nine to 10 years," he said.

'Potential pitfalls'

At a conference on Thursday on extending the school day, the president of the Association of School and College Leaders, Sue Kirkham, will give a warning about the impact of charging parents for extra-curricular activities.

"Most secondary schools are already providing breakfast clubs and other extended activities for their students and local communities and many are working with other agencies to provide extra support for families," she is due to say.

Her union's members are "enthusiastic about the possibilities".

But Ms Kirkham will also say there are "huge potential pitfalls".

'Vital'

"There is still insufficient clarity about funding, charging for activities and the roles of school leaders and governors.

"We do not want separate funding streams for extended activities, but the additional money to fund these initiatives must be clearly identified within school budgets."

The funding already committed - which is �840m between 2003 and 2008 - is a sign of goodwill but is "inadequate in the long term", she will say.

Education Minister Beverley Hughes is also due to address the conference, which is co-hosted by the National Governors' Association.

In a statement, her department said extended schools were "a vital part of our reforms to improve the lives of every child and raise school standards".