By Michelle Nichols

UNITED NATIONS Feb 5 (Reuters) - The United Nations special envoy to Somalia is optimistic that the country's parliament will approve the prime minister's new cabinet next week, allowing preparations to begin for a new constitution and presidential elections in 2016.

In December, President Hassan Sheikh Mohamud named his third prime minister, Omar Abdirashid Ali Sharmarke, after a row with the previous premier.

Parliament then rejected Sharmarke's first cabinet amid complaints from lawmakers that 10 of the 25 ministers first chosen were politicians who had been in the previous government that had not delivered change fast enough.

"I am optimistic actually that the list on Monday will be a broader more representative one, which hopefully the parliament will feel able to approve," U.N. special envoy Nicholas Kay told Reuters at the United Nations.

Only when a new cabinet is in place can it name an independent election commission, which parliament must approve.

"Without that commission in place there will be no elections and that's one of the bits of business which has been caught up and delayed in this political crisis," Kay said. "We do hope before parliament goes into recess in the next week or so that they'll pass that law ... and also appoint commissioners."

The goal is to put a constitution to a referendum in March 2016 that will outline a new federal structure to help overcome the regional and clan rivalries that have fueled past fighting. A vote for a new president is due to follow in September 2016.

"Every day counts. There is absolutely no room for slippage at all now," Kay said.

Somalia is slowly recovering from two decades of conflict. A military campaign has driven Islamist rebels out of major strongholds and some refugees have begun returning home, but efforts to rebuild the state have stalled.

(Reporting by Michelle Nichols; Editing by Alan Crosby)