President Mohamed Morsi must have set some kind of record for the speed of his descent from hero to villain. Not 48 hours ago, Egypt's President was fielding tributes from all over the world for his adroit and responsible diplomacy. He was credited with brokering the truce that had brought an end to a week of escalating violence between Israel and Gaza. Extravagant hopes were voiced that he had paved the way for a new age of moderating Egyptian influence in the region.

Within hours, all those hopes were dashed – and not because the truce had failed. For all its fragility, it was holding, to the relief of those on both sides of the Israel-Gaza border. No, the reason why such widespread optimism had turned so quickly to indignation in Egypt and something akin to despair abroad was entirely of Mr Morsi's own making.

In what looked like a profoundly misguided attempt to capitalise on his international acclaim at home, the Egyptian President passed a decree in which he gave himself extensive new powers. They include the stipulation that presidential decisions cannot be revoked by anyone, including a judge. The decree also appears to make possible new trials of those already convicted of killings during the 2011 uprising. Those liable to face new court proceedings could include the overthrown President, Hosni Mubarak.

It is hardly necessary to count the ways in which Mr Morsi's move is undesirable and plain wrong. The head of Egypt's lawyers' syndicate stood alongside leading opposition figures to denounce the presidential decree as bringing about "the total execution of the independence of the judiciary". In a colourful phrase, Mohamed ElBaradei accused Mr Morsi of appointing himself Egypt's "new pharaoh". Other critics called the decree "a coup against legitimacy". It is difficult to find fault with any of these descriptions.

By yesterday afternoon, protesters were back on the streets of Cairo. Offices of the Muslim Brotherhood were ablaze in the restless cities of the Nile delta. The favoured slogan was "Morsi is Mubarak", which summed up not only the repressive nature of the President's decree, but the very real dangers it represents, too. If Mr Morsi believed he was establishing a streamlined system of rule with a view to accelerating his country's transition to a modern, post-Mubarak, era, as some of his statements seemed to suggest, he is grievously mistaken.

In the benevolent spotlight Mr Morsi enjoyed following the announcement of the Gaza truce, it was possible to glimpse a new Egypt. An Egypt in which entrenched fears about the nature and intentions of the Muslim Brotherhood were shown to be unfounded. An Egypt able to harness a distinguished tradition of scholarship and diplomacy to build its future. An Egypt ready and able to play a positive and calming role in an unstable region. An Egypt that other powers, starting with the United States, would treat as a partner, with respect. An Egypt whose President had an electoral mandate and whose elected legislature was in the early stages of drafting a constitution. All these changes were seen – inside and outside Egypt – as the hugely positive fruits of the 2011 uprising.

With the presidential decree, much, if not all, of that has been overturned. Any attempt to retry those already convicted turns back the clock needlessly. Television footage of the former President being wheeled into court on a hospital bed do not need to be revisited. Latent suspicion of the Muslim Brotherhood and its intentions will resurface. Until yesterday, the signals from Egypt were mixed, but generally more positive than negative. Now, the balance is decisively reversed. If he wants the new Egypt to flourish, Mr Morsi must reconsider his decree.