As fighting has raged, pictures of Hamas leaders out of danger and enjoying the high life while their subjects suffer have gone viral. But according to Israeli expert Moshe Elad, who has been closely watching the Islamist movement for years, the images only show the tip of the iceberg.

He estimates that around 800 Hamas affiliates have become millionaires by circumventing the Israeli blockade and importing goods via secret tunnels running from Egypt.

Although the tunnels economy is now in crisis due to Egyptian crackdowns, tycoons have made fortunes, as have the Hamas officials who have gathered taxes on goods imported through underground passages.

But this is nothing compared to the organisation's leaders, such as Khaled Mashaal and Mousa Abu Marzook, who are estimated to be worth $2.5 billion (£1.4bn) each.

Dr Elad was head of Israeli security co-ordination with the Palestinian Authority in the West Bank in the mid-1990s, and says that he has seen first hand how corruption makes Palestinian political leaders rich. "I was shocked, stunned, to see that what really bothered them was their business," he said, adding that Hamas has replicated the same corruption model.

He added: "They came to power to implement their ideology but they were very much aware that in Gaza you can make money."

The Egyptian media has been highly critical of Hamas's corruption. One TV presenter showed pictures of Mashaal relaxing in a Qatari gym, and declared: "If you're a man and a hero, tomorrow get on the first plane and enter Gaza through the Rafah crossing."

There is also a track record of donor money to Gaza disappearing or getting swallowed up in bad investments.