International investigators have amassed the strongest evidence “since Nuremberg” for the prosecution of Bashar al-Assad and his allies for war crimes, smuggling 600,000 pages of official documents out of Syria.

This trove - weighing several tons - includes the records of a secret committee of security chiefs placed in charge of crushing the revolt. Another 500,000 pages are still inside Syria, awaiting safe transit out of the country.

The evidence is being held in an undisclosed European city by the Commission for International Justice and Accountability, an organisation of lawyers and investigators partly funded by the British Government.

The most striking evidence concerns Assad’s response to the mass protests against his rule that swept Syria from 2011 onwards. He appointed a “Central Crisis Management Cell” and gave the security chiefs on this committee supreme responsibility for suppressing the unrest. The cell held daily meetings in Damascus, chaired by Mohammad Said Bekheitan, the second most senior member of the ruling Ba’ath party.

But the 24-year-old official who kept the committee’s records and transmitted its orders, Abdelmajid Barakat, was secretly working for the opposition. In 2013, he escaped from Syria into neighbouring Turkey, taking as many of the cell’s documents as he could carry.

The paper trail shows that Assad himself “reviewed the proposals [of the cell], signed them, and returned them for implementation,” according to the New Yorker, adding: “Sometimes he made revisions, crossing out directives and adding new ones.” Mr Barakat was “certain that no security decision, no matter how small, was made without Assad’s approval.”