Kenyan President Mwai Kibaki has been told if he campaigns for a "yes" vote then November's planned referendum for a new constitution will be a farce.
Kenyans will choose a banana for "yes" and an orange for "no"
Electoral Commission head Samuel Kivuitu said no-one could compete with the government and their allies.
"The people should be left to decide for themselves," he said.
The proposed new constitution has been criticised by the opposition and five members of Mr Kibaki's cabinet.
They say it leaves the president with too much power.
Mr Kibaki has already given his support to the document.
The referendum is to be held on 21 November with voters asked to approve or reject an amended version of a draft constitution recently passed by MPs.
It is feared campaigning could further widen rifts within the ruling Narc coalition.
Violent protests took place in July over the alleged failure of the new document to limit presidential powers.
THE TWO DRAFTS
Previous draft envisaged:
Executive prime minister sharing power with the president
Four-level devolution: national, provincial, district and rural
Two chambers of parliament
Executive president and non-executive prime minister
Two-level devolution: national and provincial
Only one chamber: the national assembly
The ballots will actually ask: "Are you for or against the ratification of the proposed new constitution?"
Voters will select the symbol of a banana if they back it and an orange if they do not, election commission head Samuel Kivuitu told reporters.
The review process started towards the end of President Daniel arap Moi's 24-year tenure, but President Kibaki swept to power in 2002 promising a new constitution within 100 days, including the post of prime minister.
The government hopes to enact the constitution by 12 December, if it is passed, which will be almost three years since Mr Kibaki took office.
It will be the first major overhaul of the country's constitution since Kenya's 1963 independence from Britain.
Critics of the draft say it fails to establish a strong prime minister's post, which they say would prevent the president abusing his powers.
Instead, the premier is appointed and can be dismissed by the president.