Sir Bradley Wiggins could be ruled out of the defence of his Tour de France title by a knee injury that is threatening his place in the Team Sky squad for the race. Dave Brailsford, Team Sky's principal, told the Guardian on Friday morning that Wiggins is currently unable to train properly because of the injury.

The 33-year-old triple Olympic gold medallist has an inflammation in his left knee, which could be due to having too much play in his pedal, and has been unable to train as he would have wished.

Brailsford confirmed that Wiggins has yet to return to full training. "With these setbacks I can confirm that Brad cannot currently train properly and we are monitoring this situation carefully," he said. Wiggins has not been announced in Team Sky's squad for the Dauphiné Libéré stage race which begins on Sunday.

The injury is not related to Wiggins's crash on stage seven of the Giro d'Italia, when he fell on a descent en route to Pescara, and was seen afterwards treating apparent bruising on his right knee with ice. However, the Londoner did begin complaining of the knee problem on the rest day of the Giro, 36 hours after the race's first long time trial, and the day after a rainy and cold stage through Tuscany.

The injury is clearly not severe enough to bring Wiggins to a complete halt, as once he had recovered from the chest infection that curtailed his attempt to win the Giro 12 days into the race, he was able to resume riding his bike. However, he has not been able to get back into the intense training that would be necessary for him to compete for a high overall placing in the Tour. It is still unclear when that will happen.

His possible withdrawal from the Tour would solve a conundrum for the management at Team Sky, which will be led at the French race by Chris Froome, the Kenyan-born Briton who finished runner-up to Wiggins in the 2012 race. Froome has enjoyed a successful 2013, winning the Tour of Oman, Critérium International, and Tour of Romandie.

Since the start of the season, Froome has been Sky's leader-in-waiting for the Tour, with Wiggins expected to assist him in the quest for the yellow jersey. But Wiggins had also made it clear that he would like to try and win the Tour for himself if he were in better form than Froome, which raised the prospect of internal conflict within the team. If Wiggins does not start the Tour, Sky's back-up leader could be the Australian Ritchie Porte, who won the Paris-Nice stage race in March.

2013 is turning out to be an annus horribilis for Wiggins, who enjoyed a spell of perfect form from late May 2011 to August 2012, in which time he won the Dauphiné twice, took the silver medal in the time trial world championships, finished third in the Tour of Spain, and won the Tour de France, Paris-Nice and Tour of Romandie, capping the run of success with the gold medal in the time trial at the London Olympics.

By the end of 2012, he admitted that he had lost his way a little due to the commercial pressures that had resulted from his great season, and he did not begin training again for 2013 until mid-November, after a brief break to recover from being knocked off his bike by a van near his home in Lancashire.

Wiggins was not at his best in his first event of the season, the Tour of Oman, and finished fourth at his second race this year, the Tour of Catalonia. At his final warm-up event before the Giro, the Giro del Trentino, he had a mechanical problem on a key mountain stage, while at the Giro, he suffered in the cold and wet conditions, and lost time after his crash on stage seven, while a slow puncture cost him more time in the next day's flat time trial.

A few days later, he was clearly unwell and was on antibiotics with a chest infection, which hit him hard on stage 12 to Treviso, where he was dropped from the bunch on flat roads, pulling out of the race the next morning.

Without a ride in the Tour, Wiggins would have two chances to redress the balance. He is now likely to build for a tilt at the Tour of Spain which runs from late August until mid September, and that will be followed by the world time trial and road race championships, both of which are races he has declared an interest in targeting.