By Daniel Schweimler BBC News, Buenos Aires

Speculation has been rife for months over who will run

Her official campaign will be launched later this month. Opinion polls suggest a victory in the first round of voting.

Speculation remains as to why this decision has been taken.

Some rumours suggest that Mr Kirchner is ill, while others say a series of defeats in recent local elections have forced a change of emphasis.

It has been the main topic of speculation in Argentina for months: Nestor or Cristina, Mr or Mrs.

Now we know that Nestor Kirchner, who has governed Argentina for the past four years, will step aside to allow his wife to stand for president on 28 October as the ruling party's candidate.

The glamorous and well groomed Cristina presents a very different image from the rather dour Nestor.

She recently toured Europe and South America in what correspondents say was an exercise in preparing the first lady, already a state senator, for higher office.

If elected, she will not be the first woman president of Argentina.

President Juan Domingo Peron's widow, Isabel, led the country briefly in the early 1970s while his second wife, Evita, although never president, formed a formidable ruling partnership with her husband in the late 1940s and early 50s.