Unsurprisingly, after six Premier League titles, two FA Cups and one Champions League triumph during his 10 campaigns at Old Trafford, the 36-year-old still believes the Red Devils will never be overtaken by its big-spending neighbours.
City finished third in the Premier League last season, nine points behind United, to qualify for the Champions League for the first time.
But while Beckham acknowledged Roberto Mancini's side have developed rapidly in recent years he cannot see them usurping United.
"They're bringing good players in, they've got strong people with a lot of money behind them, so they're going to improve," Beckham told Sky Sports News.
"But they'll never be Manchester United. They might be a threat this season but United have got the history, the silverware over the last 20 years.
"There's only one team in Manchester but City I'm sure will be a team to look at going forward."
Beckham also does not believe the potential loss of Carlos Tevez, who still wants to leave after a transfer to former club Corinthians fell through yesterday, will harm City too badly.
"Tevez was a really strong player for them last season, so if they lose him it will be disappointing," he added.
"But they've got a lot behind them as an organisation, so they can afford to bring world class players in.
"If they continue to do that I'm sure they'll be strengthened."