Known as the Building Bridges Initiative, it has already been rejected by the High Court and the Court of Appeal as unconstitutional.

The attorney-general and two others appealed over four main issues, including whether a sitting president can initiate a constitutional amendment process without going through parliament.

The debate over the initiative is closely linked to the battle to succeed President Uhuru Kenyatta, who is due to step down in August after serving two terms.

Critics see the proposal to expand the executive arm of government as a way to reward politicians.

President Kenyatta and opposition politician Raila Odinga support it, arguing that they will end the winner-take-all structure of Kenyan politics.

Deputy President William Ruto, who has fallen out with Mr Kenyatta, opposes the initiative.