During wiretapped conversations the men discussed an attack on Charles De Gaulle airport outside Paris and spoke of the need to "strike at the British", Italian police said.
Bassam Ayachi, 62, a Syrian imam with French citizenship, and Raphael Frederic Gendron, 33, a Frenchman who converted to Islam, were allegedly part of an al-Qaeda cell operating in Europe.
They have been in prison in Bari, a port town in southern Italy, since November, when they were arrested on suspicion of smuggling five illegal immigrants into Italy aboard a camping trailer.
Ayachi is a well-known extremist preacher based in Belgium and mentor to Malika el-Aroud, a leading female figure in al-Qaeda whose first husband was killed in an attack against Ahmed Shah Massoud, then leader of the anti-Taliban Northern Alliance.
El-Aroud was arrested in Belgium a month after Ayachi and accused of planning a suicide attack while Prime Minister Gordon Brown was attending a meeting of European leaders in Brussels.
Italian officers placed a listening device in the cell shared by Ayachi and Gendron.
In one bugged conversation, the pair discussed "striking the British" and launching a Sept 11-style attack using an aircraft, although the target was not specified.
"They are key figures in al-Qaeda's European organisation," said Giorgio Manari, the chief of police in Bari. The wiretap evidence had enabled investigators to "nip the plot in the bud", he said.
The men were served warrants charging them with criminal association linked to international terrorism and will remain in custody.
They were also suspected of recruiting militants for suicide attacks in Iraq or Afghanistan.
Police said the pair had for years lived in Belgium, where Bassam was an imam at an extremist Islamic centre and one of al-Qaeda's "spiritual guides", while Gendron, a computer expert, was the "media propaganda point man, via the internet, for the French-speaking community."