Cherie Booth, wife of Labour leader Tony Blair, was at the centre of a political row following an article about school governors in The TES last week in which she called for greater powers for local education authorities.
Paddy Ashdown, the Liberal Democrat leader, accused the Tory party of running a smear campaign against her after Conservative chairman Brian Mawhinney issued a statement condemning Ms Booth's comments. He also wrote to Mr Blair claiming Ms Booth was now setting Labour party policy.
At Question Time on Tuesday Mr Ashdown challenged the Prime Minister to distance himself from Tory party "personality attacks".
"Will you now tell us whether you personally approve of the unpleasant campaign being run by your party chairman to attack the Labour party through the Labour leader's wife?" he said.
Mr Major hotly denied there was any such campaign, but Conservative chairman Dr Mawhinney later hit back with an angry letter to Mr Ashdown. He insisted Ms Booth was fair game and claimed her article outlined a new development in Labour's education policy.
A Liberal Democrat spokeswoman said: "There are lots of weak links in Labour's election campaign but Cherie Booth is not one of them."