John Major has stepped up his effort to secure the Tory leadership for Ken Clarke with an article warning the party of the risks of a victory for Iain Duncan Smith.

Wriiting in the current edition of "The Spectator", Major says Clarke has the necessary attributes to face down Tony Blair's government and raises serious questions about Duncan Smith's leadership credentials.

The former prime minister says Clarke is "pugnacious and lucid and will biff the prime minister with the same superb vigour as William Hague''.

"Ken has a formidable intellect combined with a powerful bite. He will unleash both on Tony Blair, defeat Labour and bring the Conservative Party home to where it belongs. On the centre-right. In government," he writes.

Whist Major accepts that Clarke is "stubborn, often infuriating, and very argumentative" he is "above all, a man of honour, decency and personal integrity".

As Tory hostilities intensify, the former prime minister says Duncan Smith lacks the voter appeal necessary to return the Tories to government.

Major warns Tory supporters that the euro-sceptic right is using Duncan Smith to further its own extremist cause.

"He should be warned: they are not pro-Duncan Smith, they are anti-Europe and thus anti-Clarke. He should ditch them speedily for they are electoral poison. No Conservative who is seen as too close to them, or as their spokesman, or worse, their captive, could be elected prime minister," writes Major.

Whilst he concedes that Duncan Smith has "engaging old-fashioned courtesy" he says it is a "misfortune" that his support base is comprised of "the active and enthusiastic support of the wilder europhobes in and out of parliament".

In his article, Major also slams the "crude innuendo and tasteless'' way Michael Portillo was "forced" out of the leadership campaign.

Referring to Lord Tebbit's description of Duncan Smith as a "normal family man", Major writes: "This was crude innuendo - with a nudge and a wink - to draw attention to Michael's past in the hope that it would harm him."

However, he concedes that some of Portillo's supporters were involved in negative campaigning. "Not that the Portillo camp has entirely clean hands: some of his lieutenants themselves were not averse to negative briefing but indulged in nothing so odious as the tactics used against Michael," says Major.

Blaming a "lethal cocktail of boredom, sleaze and splits" for the Conservatives' electoral collapse Major said that future Tory disunity would cause "incalculable" losses to the party.