David Cameron conceded that bigger spending cuts would be needed in other departments after he announced a commitment to ring-fence England's schools budget.

The Prime Minister admitted that the move would leave a future Conservative government with fewer options as it seeks to eradicate the UK's budget deficit by 2017.

Defence and welfare are among the areas that could face tighter spending restrictions because of the latest Tory pledge.

But Mr Cameron also faced accusations that he was trying to pull the wool over voters' eyes, as he was forced to admit that the amount spent per pupil in England would actually fall in real terms, because it would not keep pace with inflation.

However, the amount spent overall on schools is set to increase following a baby boom.

Mr Cameron made his commitment during a speech at a school in North London.

In a Questions and Answers session afterwards - in which he refused to answer a simple multiplication question - he admitted that his decision would affect the budgets of government departments over the next five years.

"It does mean making difficult decisions elsewhere," he said.

But he said that since the coalition came to power in 2010 it had shown that "in some areas of Government spending you can do more for less".

Mr Cameron also faced claims that he was not really protecting schools funding per pupil.

Labour claimed that the Prime Minister had let the cat out of the bag.

To add to his woes, the highly respected Institute for Fiscal Studies, Paul Johnson, said that Mr Cameron's pledge represented "quite a watering down" of the "ring-fence" protection offered to schools by the coalition government over the past five years.

The row came as Labour hit out at the boss of Boots, who warned at the weekend that voters should not back a future Labour government.

In a significant stepping up of the party's rhetoric Labour leader Ed Miliband said that Monaco-based Stefano Pessina should stop lecturing voters and "start paying his taxes".

But Labour were also drawn into a row over education as the shadow chancellor Ed Balls dismissed warnings that Labour would inflict "colossal" damage on universities in England by cutting tuition fees from £9,000 to £6,000, a move that could have serious knock-on effects in Scotland .

There was uproar among many in the universities sector in Scotland in 2010 when the Coalition slashed funding to institutions at the same time as it increased fees in England to £9,000.

Mr Miliband also faced criticism for saying that he had been an advisor at the Treasury when asked about his experiences of work outside politics.

The Labour leader was being questioned by teenagers live on air as part of an event hosted by the social networking site Facebook and Sky News.

As part of the same discussion voters were told they should write rude words on ballot paper if there is no candidate who represented their politics, by Natalie Bennet, the leader of the the Green party leader in England.