Faraj al Swehli, commander of the Swehli brigade, said his men had found official Libyan documents and equipment used by the Israeli military among their possessions.
There was also footage of them firing weapons.
Al Swehli told a press conference the men would be handed to the Libyan intelligence service who would "pursue the legal process against them".
The pair have been named as Nicholas Davies and Gareth Montgomery-Johnson. They were working for Iran's state Press TV when they were detained on February 22 in Misrata, about 130 miles (200k) east of the capital.
They are now being held in a Swehli brigade base in central Tripoli.
A rebel fighter in Misratah
Rebel fighters in Misrata where the two Brits were detained
Al Swehli showed a grey plastic packet containing a field dressing with the words "Made in Israel" on it, which he said had been found on the two journalists.
He also said the two had in their possession Libyan documents listing members of a Tripoli militia killed in a clash with a rival group late last year.
An official from Misrata said investigators had also found pornography on the Britons' laptops but that it could not be shown for reasons of decency.
Other footage appeared to show the journalists late at night in Tripoli's Algiers Square, dancing to Western pop music coming from a car's sound system.
The militia holding the two Britons is one of dozens in Libya which helped topple Gaddafi but are now operating beyond the control of the new national authorities. They are ignoring government requests to disarm.
Al Swehli said the two men were being well treated and that they had been visited by British consular officials and representatives from New York-based Human Rights Watch.
A spokeswoman for the Foreign Office said it was providing consular assistance to the two men. She declined to comment on the spying claims.
Press TV is based in Tehran and broadcasts around the world in English. Many of its journalists are from English-speaking countries.