Webb set to miss Six Nations
A Welsh Rugby Union statement said Webb had been released from the Wales squad with a knee injury that would "restrict" his involvement in the Six Nations.
The 29-year-old has now been replaced in Warren Gatland's squad by the uncapped Tomos Williams.
Wales could be without as many as nine senior players when they begin their campaign against Scotland in Cardiff on February 3.
And they are likely to still have injury problems when they face reigning champions England at Twickenham the following week.
British and Irish Lions stars Jonathan Davies, Sam Warburton and Dan Lydiate had all been previously ruled out of the entire Championship, while lock Jake Ball could miss all five fixtures as well with a shoulder problem.
And key figures such as No.8 Taulupe Faletau (knee) and flyhalf Dan Biggar (shoulder) are in line to miss the opening three rounds of the tournament, which includes Wales's match against Ireland in Dublin on February 24.
Biggar's fellow flyhalf Rhys Priestland is currently sidelined with a hamstring problem, while Liam Williams may need to undergo surgery on an abdominal injury.
The 26-year-old utility back – who played in all three Tests of the Lions' drawn series with world champions New Zealand last year – is having extensive rehabilitation treatment in a bid to be available for the clash with a resurgent Scotland at Cardiff's Millennium Stadium.
Frustrated Wales coach Gatland implied Williams would have been fit if his English club team Saracens had him to undergo surgery when the same injury sidelined him for six weeks following the Test win over Georgia in November.
Williams – who normally plays on the wing for Wales though he played fullback for the Lions – felt a recurrence when he returned for reigning title-holders Saracens in their European Champions Cup draw with Welsh pool rivals Ospreys earlier this month.
"It's probably an injury that six weeks ago he should have had an operation on it, but Saracens had rehabbed him and he's come away from the game against the Ospreys feeling very sore," Gatland told reporters during Wednesday's official launch of the Six Nations in London.
"Potentially he had the option of having surgery before but that wasn't taken," added the New Zealander, looking to guide Wales to a first Six Nations title since 2013.
"Our medical team had a look and want to rehab him extensively over next couple of weeks."
Meanwhile, Webb's injury could signal the end of a Wales career that has seen him capped 31 times by his country as a result of the Welsh Rugby Union's new "senior player selection policy".
Introduced in October, this states that no player with fewer than 60 caps to his name will be eligible for Wales selection if they are playing their club rugby outside of the Principality.
Shortly prior to the WRU's announcement Webb – who later insisted he'd been unaware of the looming change in policy – revealed he had signed a three-year contract to join French giants Toulon from Welsh regional side Ospreys at the end of the current European season.