Swiss brothers Urs and Marco Tinner - suspected of involvement in nuclear smuggling – will file a complaint with the European Court of Human Rights next month.
Their lawyer said Switzerland had disregarded the European Convention on Human Rights by keeping the engineers in custody for nearly four years without an arraignment.
Confirming a report in the NZZ am Sonntag newspaper, the lawyer also said the Swiss government's decision earlier this year to shred documents seized during investigations was in violation of the convention.
He said this was a massive intrusion into the case.
The lawyer's announcement comes after the Swiss Federal Court in August upheld a ruling ordering the Tinners to remain in custody pending the outcome of the investigation.
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.
The thee are known to have 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.