Unions are also calling for the reforms to be properly funded

NUT general secretary Steve Sinnott said it would "alienate and undermine" any desire disaffected youngsters may feel towards continuing in education.

The leader of the NASUWT, Chris Keates, said compelling pupils to stay in education was unlikely to work.

The government says only a small number of youngsters would be prosecuted.

Students who leave at 16 usually are those who don't do well in a traditional classroom Association of School and College Leaders Learner drop-outs could be fined

Mr Sinnott said: "The government appears to want it both ways: voluntary involvement in education and training post-16 but being criminalised if you don't take part.

"What is needed is for the government to talk to teachers' and employers' organisations and to young people's groups about the sort of provision that should be available and then ensure that happens."

Ms Keates said the rationale and vision underpinning the proposals was undoubtedly right, but that forcing youngsters to stay on in education or training was unlikely to work.

"It is unlikely that a change in the law will change the minds and attitudes of youngsters who are disinclined to stay on now.

"This is a duty which should rest firmly with local authorities and not with individual schools."

She added: "Raising the leaving age will, however, have significant implications for the workforce in schools and further education.

"Staffing, provision of targeted support and the capacity of existing facilities are obvious issues to be addressed. With appropriate funding and carefully planned implementation these are unlikely to be insurmountable issues."

'Substantial burden'

Association of School and College Leaders head John Dunford said it was vital the government's "lofty aspirations" were properly resourced.

Dr Dunford said: "Students who leave at 16 usually are those who don't do well in a traditional classroom and cannot see the connection between their studies and improving their earning power.

"To have real benefit for the economy and, more importantly, for young people, they must have access to education and training that interests and motivates them."

He added that the new specialised diplomas, being introduced from 2008, were not a "cheap option".

"If we are to reach 100% participation to age 18, this will place a substantial financial burden on the system, in the further education sector in particular.

"Government and employers need to be clear and consistent about what they really want from the education system, and to provide resources as well as targets."

'Options'

The director-general of the British Chambers of Commerce, David Frost, said: "This will give both individuals and businesses the necessary skills to drive the UK economy forward in the face of continuing globalisation."

TUC general secretary Brendan Barber said it made sense for teenagers to remain in education or high quality training until they were 18.

"But these proposals must be backed by proper resources for the education sector to ensure that there is sufficient high quality provision that appeals to young people with a variety of interests.

"Employers are going to be important in making this vision a reality and the TUC looks forward to working with ministers on these proposals."

The chair of the Local Government Association's board for children and young people, Councillor Les Lawrence, said a "broad range of options" needed to be available to them.