Iain Duncan Smith: "Intensely serious"

Mr Duncan Smith was elected leader of the party after defeating Ken Clarke in the race to replace William Hague.

Sir Malcolm said that where the public considered former leader Mr Hague as lacking in gravitas, the new leader "if anything has too much. He is a very serious figure".

Sir Malcolm writes: "He comes across as an intensely serious person.

"He looks worried all the time. He chooses his words very carefully.

'Party opinion'

"That creates some problems for him in terms of charisma, in terms of excitement and personality, but what it doesn't do is create an impression of someone who is frivolous and the public probably like that in a potential prime minister.

"How decisive that will prove in terms of the overall public judgement is it too early to say."

Sir Malcolm goes on to say that it is in Mr Duncan Smith's interest to achieve unity in the party.

He writes in Scotland on Sunday: "That means acting in a way that will attract the support of the whole spectrum of party opinion.

Like Labour, the Tories will not win from simply one end of the political spectrum

Sir Malcolm Rifkind

"That has been done in the past by all our successful leaders so I was more than surprised by the shadow ministerial appointment of Bill Cash: I was surprised and disturbed.

"I understand it was made partly to shut him up as far as public comments on Europe are concerned.

"If this is the case the public will be well served, but I feel sorry for the shadow cabinet who will have to continue listening to him.

"Whoever is leader of the party has to lead a party in which all points of view are represented and that must be done in substance, not just in symbols.

"Like Labour, the Tories will not win from simply one end of the political spectrum."