Ruddock was set to name Parker in his squad for the 2006 RBS 6 Nations, with Wales starting the defence of their Grand Slam title against England at Twickenham on February 4.
Wales were already without injured stars Brent Cockbain, Ryan Jones, Chris Horsman, Tom Shanklin and the suspended Gavin Henson for their opening game.
Parker had played a part in last year's RBS 6 Nations victory and was expected to feature in the title defence but has now decided to concentrate on his career with Ospreys.
"We are incredibly disappointed that Sonny has decided to step down from the international stage and made every effort to dissuade him from doing so," said Ruddock.
"We would have picked him in this year's squad as we prepare to defend the championship title. But we have to respect his decision when he says that he can no longer give the commitment required for rugby at this level."
Twenty-eight-year-old Parker began his rugby career in Europe after leaving his native New Zealand for Italy and Viadana as a 19-year-old and went on to play for Pontypridd, Ospreys and Wales.
"This is one of the biggest decisions a player will ever make, but it is just something I have to do," said Parker.
"I've come to the stage in my career where I only have around three seasons left to play rugby and I want to concentrate on the career I will have when I stop playing while giving all I can to the Ospreys."