Hamza and other fanatics based at the Finsbury Park mosque in London, were suspected of having links with Rachid Ramda, who was jailed for involvement in the 1995 Paris Metro bombings.
Amid a long-running battle to extradite Ramda from Britain, the French referred to the capital as "Londonistan" because of its reputation for sheltering Muslim terrorists.
The French plot was contained in an in-depth report about the now banned extremist group al-Muhajiroun, which Hamza had links with.
The report, Gateway to Terror, claims the group was the "biggest gateway to terrorism in recent British history".
It was co-written by Nick Lowles, the chief executive of Hope not Hate, a former editor of the anti-fascist magazine Searchlight.
On the Hamza plot, the report said: "The plan was to impersonate the British Nazi group Combat 18 and then allow them to take the blame.
"In the immediate aftermath of the London nail bombings, the French considered sending death threats in the style of C18 and then killing him with the same type of weaponry that the group was thought to possess.
"However, as with the kidnap plan, the assassination attempt was aborted and it is unclear how advanced the planning of either idea progressed."
Hamza was last year extradited to America to face terrorism charges.
The report claims 70 people convicted of terror-related offences or known to have died fighting overseas had a link to al-Muhajiroun.