EGYPT'S army had no choice but to overthrow Islamist president Mohamed Morsi, former British prime minister Tony Blair says.

Writing in Sunday's Observer newspaper in Britain, Blair, who acts as Middle East envoy for the United States, Russia, the EU and the UN, said the army's only alternative would have been to let Egypt descend into chaos.

"The events that led to the Egyptian army's removal of President Mohamed Morsi confronted the military with a simple choice: intervention or chaos," wrote Mr Blair, who led Britain for a decade from 1997.

The army toppled Morsi from power last Wednesday following huge protests against his rule. But dozens of people have been killed in clashes in the wake of his departure.

Mr Blair said the West had strong incentives to support Egypt's democratic transition.

"At its crudest, we can't afford for Egypt to collapse. So we should engage with the new de facto power," he wrote.

"In that way we can also help shape a path back to the ballot box that is designed by and for Egyptians."

Likewise, Blair argued that Western nations have a responsibility to work towards peace in the wider Middle East.

"We feel it should be someone else's job to help sort it out. But it is our job," he wrote.