Schools are struggling to provide the additional catch-up tuition in English and maths they have been funded for because of difficulties in recruiting tutors, Government-commissioned research shows.
Both Labour and the Liberal Democrats have made extra one-to-one tuition key points of their general election manifestos, but new figures show that the proportion of pupils getting the promised help is way below the Government target of 10 per cent.
The study, by PricewaterhouseCoopers, shows that it could be hard for schools to deliver even when the money to pay for it is there.
The consultants found that around 190 of the 450 primary and secondary schools funded for extra tuition through the Government's Making Good Progress pilot were unable to recruit enough tutors.
They cite these "ongoing challenges" in recruitment as an explanation for the tuition only being delivered to a maximum of 2.5 per cent of pupils.
The figures are for 200809, the second year of the two-year pilot. At the end of the first year the rules were relaxed to allow the extra tuition to take place during school hours, to try and ease the problem.
This meant some pupils missednormal lessons, including a minority in the core subjects of English, maths, science and ICT.
But despite this, the study reports that in the second year, 42 per cent of heads said they had still been unable to recruit enough tutors in maths and 44 per cent said the same for English.
Labour's manifesto highlights the party's "3Rs guarantee of one-to-one and small-group tuition for every child falling behind".
The Liberal Democrats suggest that schools will want to use their portion of a #163;2.5 billion pupil premium to pay for "extra one-to-one tuition".
John Bangs, NUT's head of education, said: "This does raise real questions about how you get these individual tutors.
"It is not that there are not enough teachers out there. There are lots of supply teachers who can't get work because of the school workforce agreement.
"But the basic structure and mechanism to help schools find these teachers was never put in place. No one has organised and trained for it."
The study found that schools were reluctant to employ tutors from agencies because of "burdens around administration". But they also had quality concerns with agency staff.
Where tutor recruitment was most successful, heads said, it was due to existing staff wanting to provide additional support to pupils.
A Labour party spokesperson said: "Following the initial pilots, the Government has acted to prepare the ground for the national expansion of the programme and improve recruitment of tutors, including increasing their hourly pay rate from #163;25 to #163;29-#163;33, and allowing tuition to take place during the school day."
David Laws, Liberal Democrat education spokesman, said: "Liberal Democrats are committed to spending an extra #163;2.5 billion on schools, coupled with real freedom. Schools could use this money to cut class sizes or offer one to one tuition, but a Liberal Democrat government would not direct schools how to spend the cash."