Professor Yash Pal Ghai has been fighting for a new constitution for several years

Professor Yash Pal Ghai has sent a letter of resignation to President Mwai Kibaki.

Mr Kibaki this week announced that the latest deadline for having a new constitution would be missed.

The issue has led to deep divisions in the ruling Narc coalition, resulting in the formation of a government of national unity on Wednesday.

Rival marches

In his resignation letter, Mr Ghai said he had expected the new constitution to have been enacted by 30 June.

He said the Constitution Review Commission had ended when a draft constitution was adopted in March despite opposition from Mr Kibaki's allies.

CONSTITUTIONAL PROPOSAL Elected president appoints PM PM appoints cabinet PM leads government and chairs cabinet President remains head of armed forces Threat to ruling coalition

The president had promised to introduce a new constitution within 100 days of coming to power in December 2002 but the deadline then slipped to this week.

He brought the former ruling Kanu party into the government and demoted members of the Liberal Democratic Party (LDP), who had been lobbying for a new constitution.

Before the election, the LDP's Raila Odinga was promised the new powerful post of prime minister if he agreed to back Mr Kibaki's presidential campaign.

An LDP demonstration is planned in the capital, Nairobi, on Saturday to protest against the delay.

Police have also authorised a planned rally by Mr Kibaki's National Alliance Party of Kenya on the same day, raising fears of possible clashes.

Announcing the delay in a surprise television speech, Mr Kibaki blamed a failure to agree on key issues and court cases but said he remained committed to having a new constitution.

He said that a new parliamentary bill would be introduced to debate some compromise measures.