Don Foster and his party will oppose the proposals, to be unveiled in tomorrow's Queen's Speech, on the grounds that there will not be enough funds available to pay for them.
The Liberal Democrats' manifesto promised that all primary school pupils would be taught in classes of less than 30. It said the plan, to be phased in over five years, would cost pounds 475m per year once it was implemented.
Labour says its more modest plan will cost pounds 100m, and will be financed through the abolition of the assisted places scheme, under which 37,000 pupils receive between pounds 2,500 and pounds 3,000 per year to help pay their private school fees.
However, opponents say there are two flaws with the Labour plan. The first is that the assisted places scheme will be phased out over seven years to allow pupils complete their education. The second is that the abolition of the scheme will mean more pupils move into state schools, costing around pounds 2,000 each per year.
On the plus side, the Tories had planned to expand assisted places, putting the cost up from pounds 117m to pounds 180m. That means a saving of pounds 63m which the new government can use to help fulfill its class-size pledge.
Mr Foster, the Liberal Democrats' education spokesman, will say that the Government's plans do not go far enough. His party will seek to amend the proposed legislation during its passage through the House of Commons, although with a huge Labour majority the protest is unlikely to have much success.
"Labour's plans to cut class sizes are a pale shadow of the Liberal Democrats', which would cut class sizes to 30 for all children under 11 years," he said last night.Reuse content