The two Swiss engineers, Urs and Marco Tinner, suspected of involvement in an international nuclear smuggling network have been refused bail.
The Swiss Federal Court upheld a ruling by the Criminal Court earlier this year which ordered the Tinners to remain in custody pending the outcome of an investigation.
But judges also called on the authorities to speed up the procedure or release the brothers who have been detained since 2005, according to court statement on Friday.
The pair had argued their alleged involvement could no longer be proven after the government ordered the destruction of documents on the case.
The brothers along with their father, Friedrich Tinner, are suspected of helping to supply parts for Libya's nuclear weapons programme through a trafficking ring run by Abdul Qadeer Khan, the founder of Pakistan's nuclear programme.
It is believed the three worked as undercover agents for the United States intelligence service amid speculation that the Swiss government was asked by Washington to destroy any evidence suggesting cooperation with the CIA