The news that Tony Blair’s sons were being tutored by a teacher from a leading public school was the most high-profile example yet of parental anxieties about the insufficiency of state education. The Prime Minister’s opponents cried “Hypocrite”, but he was only doing what thousands of parents do - trying to give his children an advantage in the exam stakes.

Indeed, when the Liberal Democrat Phil Willis accused the Prime Minister of “an astonishing lack of confidence in the state education system”, he was describing a trend. From middle England to the inner cities, the tutoring business is booming as middle-class worries over exams or university entrance translate into extra tuition.

Meanwhile, a grass roots movement of supplementary schools, run almost entirely by volunteers and catering for tens of thousands of pupils, is boosting achievement and giving a crucial sense of identity to children from ethnic minority groups in particular. The work of supplementary schools has gone largely unreported. They exist on small charitable grants and parental contributions. But the need for them has increased, often in response to underachievement among pupils from particular ethnic groups and the low expectations some parents feel mainstream schools have of their children. The Labour MP Diane Abbott has said the Saturday schools run by London’s black community - some dating back more than 30 years - “compensate for the failures of mainstream school”.

There were few attempts to recognise their work until the mid-1990s, when the Trust for London, a charity that funds small-scale voluntary projects, realised it was supporting about 60 disparate, community-based schools.

The trust organised a steering group to look into the phenomenon, It found a thousand. Their details were published in a directory published by the Resource Unit for Supplementary and Mother Tongue schools, the charity set up as a result of the group’s investigations The unit’s director, Mohamed Abdelrazak, was a headteacher in his native Eritrea before coming to Britain in 1991. By the time the unit published its second edition of the directory, in 2001, there were more than 2000 in existence, ranging in size from 10 pupils to more than 300. “We are talking about a huge provision,” says Mr Abdelrazak, “but it is almost invisible.”

From Japanese schools in Newcastle upon Tyne and Greek Cypriot schools in Somerset to Polish schools in Leicestershire and Nigerian schools in Liverpool, their variety is a vivid illustration of the diversity of multicultural Britain. Mr Abdelrazak likes to trace its roots back to the Armenian refugees of 700 years ago, through the arrival of French Huguenots and Eastern European Jews to the post war immigration of people from former British colonies in the Caribbean and Asia.

At a time when the need for them is growing and their profile is becoming stronger, government support is wavering. The Supplementary Schools Support Service (SSSS), sponsored by the Department for Education and Skills was founded in 2000 and is managed by the Centre for British teachers and the African Schools Association. It has funded 200 supplementary schools in the past two years with average grants of pound;5000. But its budget has been cut from pound;945,000 this year to pound;700,000 next.

It is planning to spread the lessons learnt from pilot schemes in London, Birmingham, Bristol and Manchester - but there will be much less money available. Instead, a spokeswoman for the service said “there will be a greater emphasis on extending local authority support for supplementary schools by working with a number of LEAs to gather and share good practice”.

Last year, SSSS research into the benefits of supplementary schools suggested that pupils were overwhelmingly positive - sometimes in contrast to their attitude towards mainstream schooling. Of the 772 pupils they questioned, only 12 per cent said they were bored at supplementary school - compared with 22 per cent at mainstream school. Three quarters were happy and found the work interesting. Eighty four per cent said it helped them with their mainstream work and the same proportion said they got on well with their teachers.

The presence of positive role models in supplementary schools goes some way to redressing the under-representation of black, Asian and other ethnic minority teachers in state schools, at least in the eyes of their pupils.

The role of mother tongue schools in reinforcing pupils’ sense of identity is vitally important, as research has emphasised the negative effects on self-esteem among children whose first language is disregarded.

Supplementary schools should be seen as a complement rather than a substitute for mainstream schools, Mr Abdelrazak says. “They should not be a spitting image of the school. They don’t have the resources.”

His children went to a school where 47 languages were spoken in the playground. “How are the teachers there supposed to find time to teach them all those languages?” Britain’s supplementary schools hold a “huge reservoir” of talent, he says, which could provide a partial solution to the recruitment problem. Some supplementary schoolteachers have qualified teacher status here and work in state schools from Monday to Friday. Many more have worked as teachers abroad. Unlike professions such as nursing, they have traditionally found it hard to work as teachers in the UK because of the requirements of the national curriculum or the language barrier.

With help, though, Mr Abdelrazak believes supplementary schoolteachers could find a role in mainstream schools. “We are short of teachers - we are even shorter of teachers from minority groups.

“And say there are five teachers in each of these schools, that’s 10,000 teachers. With some training, don’t you think that a tenth of them - 1000 teachers - could be usefully employed in our mainstream schools?”

For supplementary schools to prosper, they must become more closely aligned to the mainstream and help and inform one another, he believes. “Often there is very little or no contact between the two sectors and that is a disadvantage. I don’t believe you can split a child into a weekday child and a weekend child. If we have these schools as pockets of isolation we are not doing any good for the future of multiculturalism.”

Instead, he encourages parents involved in supplementary schools to become classroom assistants or governors of their children’s school. “The role of the community and the voluntary sector is very important here. It doesn’t mean that the state shouldn’t support and can’t support them.

“We have such a huge reservoir at our disposal. These schools can work miracles, all they need is the proper professional guidance and proper support.”