Shadow defence secretary Iain Duncan Smith confirmed yesterday that he will be joining the contest for the party's leadership, giving rank and file members the prospect of a right-leaning alternative to shadow chancellor Michael Portillo.

Mr Duncan Smith announced he would be standing, at the end of a day in which it had become increasingly clear he would do so.

He said: "I believe I will make a strong leader. Competition must take place."

Mr Duncan Smith, 47, said he would make a full statement on his reasons for seeking the leadership at a media briefing today.

David Davis, a former Minister for Europe who spent the last Parliament as chairman of the powerful Commons Public Accounts Committee, is also expected to confirm his candidature.

Their moves will ensure that shadow home secretary Ann Widdecombe will not be a candidate.

Miss Widdecombe said yesterday she would only stand if Mr Portillo looked like being unopposed.

She blamed lack of support from fellow MPs for her decision, stressing that it was not due to "any lack of will on my part or to any lack of encouragement from the voluntary party".

Early yesterday one of Mr Duncan Smith's supporters, Bernard Jenkin, shadow transport minister, said: "It seems extremely probable, yes he will stand. That is official, I am announcing his candidature."

Mr Duncan Smith said a little later: "Clearly it is likely that my intention will be to stand", but did not confirm his decision until later still,

Activists launched a website www.dd4leader.net backing Mr Davis's expected candidature.

Campaign coordinator Gary Ling said the aim of supporters was to persuade the 52-year-old Haltemprice and Howden MP to stand by demonstrating that he had considerable support.

Former Chancellor Kenneth Clarke has not yet announced whether he will join the fray.