Schools minister Charles Clarke backed Welsh Office proposals to use New Deal candidates to support the national literacy strategy, but said serious questions had to be answered before English schools joined in.
Welsh education minister Peter Hain launched a consultation exercise on Monday. The first new assistants are expected in the classroom in early 1999.
Mr Clarke told The TES: "We're positive towards it but there are serious practical issues that arise which we would want monitored very closely. I would be surprised if we could look at it for less than a year before being able to draw any sensible conclusions."
Ministers would want to be sure the assistants were genuinely contributing to the classroom and not diverting resources or undermining teaching standards, he said. "Obviously managing a New Deal trainee can be a lot more time-intensive for a teacher than looking after children, which is their primary responsibility."
The New Deal is open to 18 to 24-year-olds unemployed for six months and to over-25s on the dole for two years or more. A "gateway" interview helps them select one of three options - six months of training, volunteer work or subsidised employment.
Classroom work would count as subsidised employment, with schools given Pounds 60 a week plus a Pounds 750 training grant for the under-25s and Pounds 75 for older employees. The cash stops after six months - raising fears the assistants could find themselves back on the dole.
Mr Hain said assistants would be vetted by heads and could be "part of the Government's drive to upgrade literacy skills or be deployed to help hard-pressed teachers clearing up, preparing for lessons or giving pupils individual attention." Some could go on to train to become teachers.
But National Association for Schoolmasters Union of Women Teachers spokesman Jerry Bartlett said literacy support required skilled, trained assistants. He added: "We don't see how socially-excluded youngsters whose difficulties may well relate to their own literacy problems can effectively be involved. "
Clearing up after teachers would hardly occupy them full-time, and teachers would end up supervising them instead of the children, he said.
The National Association of Head Teachers raised similar concerns. General secretary David Hart said New Deal assistants would have to be able to cope with training - and should not be taken on at the expense of existing support staff.
But he said: "We must not be prejudiced by the fact they are unemployed. It may mean they have got the skills but haven't been able to find suitable employment."