The pair have contributed a combined 20 seasons to Cardiff RFC and the Blues but they will pay the price of a cost cutting mission at the Welsh region. The Blues are hoping to slash £750,000 off their wage bill.
"This is the biggest, hardest, toughest, worst decision I have had to make in my 20 years with Cardiff," said Blues chairman Thomas.
"They are two of the greatest servants we have ever had. I've never met two finer people than Martyn and Tom.
"I think they are absolute gems, who have given us everything. Tom has kept his knee together since the Lions tour of 2005 and put his body on the line year after year.
"And Martyn is a legend who richly deserves to become a centurion for what he has done for Welsh rugby. He is one of this country's finest ever players.
"But Old Father Time catches up with everybody. They are both coming out of contract and part of the cutbacks are that people who are out of contract can't be retained because other people are in contract.
"Over the last few years, and this season in particular, we have thrown the financial kitchen sink at the squad. Our squad cost for this season is £5m.
"We have gone for the best in Dan Parks and Michael Paterson and retained Xavier Rush. On paper, it's by far the strongest squad we have had.
"A lot of people have taken huge decisions to support this and we will continue to support it."
Shanklin is currently sidelined after undergoing knee surgery while Williams is expected to be involved in the RBS 6Nations.
But the British & Irish Lions duo are set to return at the tail-end of the season and will target the Magners League crown as they aim to finish their Cardiff careers on a high.
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