Directly challenging the decision by John Major to portray Labour as Brussels' poodles, a pushover for further European integration, while he is ready and eager to defend British interests, Sir Leon said: "For God's sake lay off. Let's not go down this road.
"There are no votes in it, and it will damage British interests... It is a blind alley that will not work."
He said that attempts to put "blue water" between the Conservatives and Labour would not work because the Labour Party had shown its ability to "creep up" on the Tories.
"If there is to be a Dutch auction of Euro-scepticism," Sir Leon warned, "whoever wins the election will be left with a legacy of commitment that will make the inter-governmental conference more difficult."
Sir Leon's controversial intervention in the British general election campaign reflects a recent European Commission decision to take a high profile in defending itself from expected attack during the election.
That decision and Sir Leon's attack will be most unwelcome to British ministers who are currently showing acute sensitivity to European issues - to the point of making heavy threats against Tory MPs who might be tempted to vote for Teresa Gorman's referendum Bill in the Commons next Tuesday.
But the attack, delivered during a lunch held at the commission's London office, went even further with a warning that the British could not block further European integration. Sir Leon also tried to reassure the Government that the chance of more legislation like that on the Social Chapter was very small indeed.
Sir Leon said the Government's attitude towards Europe was not so much annoying as "mystifying", showing an unjustified nervousness and lack of self-confidence.
He also said it was a "complete error" to think that talk of renegotiation of the Maastricht treaty strengthened the British hand: "Those who think that our partners would be so terrified of this prospect that they will lie down in a supine fashion because the taboo of renegotiation has been lifted are living in Cloud-cuckoo-land."
As for the view held by some senior Conservatives that Britain could stop further European integration, he said that, too, was mistaken. While there could be no coercion of the minority, equally there could be no possibility of a minority such as Britain holding back the majority if it wanted to move ahead.
Equally, if Britain always remained behind, that raised the "nightmare scenario of the last 300 years" of European history, in which Europe exerted a major influence over Britain, while Britain had no control over what was happening.
Repudiating the "myth" that commissioners were civil servants, who should not intervene in domestic political issues, Sir Leon said that that had never been the case for commission members, who had nearly all been active in politics before their appointment.
But the commission has also decided that its officials, such as Geoffrey Martin, head of the European Commission office in London, should be allowed to give interviews during the election campaign - to correct false impressions left by any of the contestants.Reuse content