Cardiff - Liam Williams was labelled the "world's best fullback" by rugby great Barry John after his starring role in Wales' dramatic 21-13 win over England.

Victory, sealed by wing Josh Adams' try two minutes from time off a pinpoint cross-kick from replacement fly-half Dan Biggar, left Wales as the only team capable of a Grand Slam in this Six Nations after they recovered from a 10-3 half-time deficit to dash England's hopes of a clean sweep.

The 27-year-old Williams, who plays alongside several England stars, including captain Owen Farrell, at Premiership Saracens was masterful in both attack and defence, where he neutered the threat of the Red Rose kicking game.

"The key man for me was Liam Williams, man-of-the-match in every sense," wrote Welsh hero John, one of rugby's greatest fly-halves, in his walesonline column.

John played in the same successful Wales side of the late 1960s and early 1970s as the outstanding full-back JPR Williams (no relation to Liam). The pair were also key figures in the 1971 British and Irish Lions team that won a Test series in New Zealand.

JPR was renowned for rock-solid defence and fine counter-attack, qualities John said Liam Williams had shown in front of a capacity crowd at Cardiff's Principality Stadium on Saturday.  

"Time and again he was calm under the English high bombs, caught the ball cleanly and, invariably, made good yards too," John said.

"His defence was flawless and he is without doubt the best 15 (fullback) in the world today."

"Whenever he has ball in hand his first intent is to run with it. When he does take players on everybody on the pitch, and indeed in the stands, is on red alert because they know something is about to happen."

John added: "Some players simply inspire those around them with their outrageous deeds and Liam is very much of that ilk for Wales.

"Defensively he is incredible, offensively he is our most creative back -- and that's saying something when you are chosen as last line of defence. But that's the impact he has upon this team."

Leigh Halfpenny, a top-class goalkicker, might have been Wales' fullback against England if he had been fit.

But the 30-year-old has not played a competitive match since being concussed following a late tackle from Samu Kerevi during Wales' win over Australia in Cardiff on November 10.

Halfpenny is, however, set to return for the Scarlets against the Cheetahs in the Pro14 on Sunday.

John, though, was adamant that Williams, who played a key role in the 2017 Lions drawn series against world champions New Zealand, should keep his place when Wales resume their Six Nations campaign away to Scotland on March 9.

"Even when Leigh Halfpenny is fit again, Liam must stay at full-back because, quite simply, he is the best option for Wales," John said.

"He demonstrated that with the Lions in New Zealand and he proved that again in this one.

"In fact, the young Halfpenny was a bit like Liam in as much as he loved to counter. Several bangs to the body have taken their toll in that respect and Halfpenny is far more of a conservative full-back these days, but it's the opposite with Liam."