"At the moment, he's the UK's greatest athlete. He is probably the greatest sprinter in the history of cycling."
Millar, who finished 76th overall riding for Garmin-Cervelo, added: "It's sad that it's not appreciated to the degree it should be. In France, Belgium and Italy, he is like David Beckham."
Cavendish revels in green jersey
Cavendish, who rides for the HTC Highroad team, is now joint sixth on the all-time list of stage winners and has won at least four stages in each of the last four years.
Belgian Eddie Merckx holds the record of stage wins with 34 but Cavendish is being tipped to overhaul the tally of the five-time Tour winner by Sean Kelly, who won the green jersey, awarded to the Tour's best sprinter, four times in the 1980s.
"It's incredible in such a short number of starts in the Tour de France to have 20 stage wins," Irishman Kelly commented.
"It's a huge record he has chalked up already. Going forward he should be able to match Merckx's record.
"He has time on his side, he's still very young and, if he's in a team doing what HTC have done this year, there's no reason he can't win three or four stages every year."
LE TOUR STAGE WINNERS*
Eddy Merckx (Bel) 34
Bernard Hinault (Fr) 28
Andre Leducq (Fr) 25
Lance Armstrong (US) 22
Andre Darrigade (Fr) 22
Nicolas Frantz (Lux) 20
Mark Cavendish (GB) 20
*individual stage wins
Cavendish, whose next big target is September's World Championships in Denmark, has said he is not concentrating on matching Merckx's haul.
"I'll just keep trying to win as many as possible," the Manxman stated. "There's not a number I'd like to win.
"Catching him [Merckx] is not an objective. I'd like to come back and consistently win at the Tour de France."
Cavendish has been linked with a move to Team Sky but was full of praise for his current team-mates after his latest victory.
Lead-out men Matt Goss and Mark Renshaw peeled off at the front with 160m to go to leave Cavendish to accelerate to the line ahead of Team Sky's Edvald Boasson Hagen.
"It's an incredible group of guys," Cavendish reflected. "They got me through this Tour and won me the green jersey."
The overall race was won by Australian Cadel Evans. The BMC rider finished 94 seconds ahead of Luxembourg's Andy Schleck, whose brother and Leopard Trek team-mate Frank was third.
Spain's Alberto Contador, who won the event in 2007, 2009 and 2010, was fifth, almost four minutes adrift of Evans.
Evans is the first man from outside Europe or the US to take the title
Evans said that he "couldn't be any happier" with his win, having finished runner-up in 2007 and 2008.
"What can I say? I've been dreaming of winning the Tour de France for the past 20 years, ever since I was a 14-year-old," he said.
"I went through some difficult periods. For two years I was really unlucky coming so close to winning but maybe it's just made it all that more special now.
"A few people always believed in me. I always believed in me. And we did it.
"To be here wearing the yellow jersey for my team, my country, a group of people around me... it leaves me a little lost for words."
Evans only won the fourth of 21 stages, but always remained within striking distance.
He becomes the first Australian - and at 34 the oldest rider since World War II - to win the Tour de France.
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