ZURICH (Reuters) - Three Swiss engineers guilty of involvement in a Pakistani nuclear smuggling ring avoided a prison sentence on Tuesday after agreeing a plea bargain with the Swiss Federal Prosecutor's Office.

Friedrich Tinner and his two sons, Marco and Urs, pleaded guilty at the federal criminal court in Bellinzona of supplying centrifuge parts and participating in the smuggling ring of Abdul Qadeer Khan, the former head of Pakistan's nuclear programme, who in 2004 was found by Pakistan to have sold nuclear secrets to North Korea, Iran and Libya.

However, after agreeing the plea bargain, the Tinner trio were handed relatively mild suspended jail sentences of between 24 and 50 months.

In a long testimony, Friedrich Tinner told the court on Monday about how he met A. Q. Khan in 1975 and used his business in the eastern city of St. Gallen to sell centrifuge parts for enriching uranium, the online edition of Swiss tabloid Blick reported.

The court also heard that Urs Tinner set up production sites in Dubai and Malaysia for producing centrifuge parts for enriching uranium, while Marco took care of supplies and book-keeping.

Swiss authorities started investigating Marco Tinner and his brother in 2004, confiscating thousands of documents. A year later they expanded investigations to include Friedrich Tinner.

In 2009 Urs Tinner said that he had helped the U.S. Central Intelligence Agency to uncover Libya's nuclear weapons programme by tipping it off that Libya was about to receive the equipment needed to make an atom bomb.

It was widely reported in the Swiss media that further investigations into the Tinners were halted after the CIA urged the Swiss government to drop them to avoid exposing the extent of the agency's activities.

Switzerland, which is not a nuclear power, is not authorized under the global Non-Proliferation Treaty to possess documents related to nuclear weaponry. (Reporting by Catherine Bosley; Editing by David Goodman)