The Poland striker was expected to follow in the footsteps of Mario Gotze in switching the Westfalenstadion for the Allianz Arena this summer, but Dortmund sporting director Michael Zorc has since insisted "we will not agree a transfer for Bayern FC this summer".
With Wayne Rooney requesting to leave Old Trafford again, the online bookmaker expect United to be next in line to land for the 24-year-old, whose contract expires next summer.
Lewandowski, who scored 24 goals in the Bundesliga and 10 in the Champions League last season, looks unlikely to see out his current deal at Dortmund, having indicated he wants a "new challenge".
Sky Bet Football Trader Chris Spicer said: "We consistently regarded Bayern as the favourites, to the point where we actually suspended betting with the move backed at as short as 1/5 last week.
"However, the news that Dortmund will not sell another star to their rivals is sure to alert various top sides around Europe and we now make United 2/1 favourites ahead of Chelsea (8/1) and Arsenal (10/1), but the market can be expected to change as other suitors inevitably show their interest."