The England striker stunned United last week by telling chief executive David Gill that he wanted to leave because of concerns about the club's lack of ambition in the transfer market.
Manager Alex Ferguson made Rooney's decision known on Tuesday, while still giving the 24-year-old forward the option to stay at Old Trafford.
But Rooney seems intent on leaving after issuing a public denunciation of the club's transfer activities on Tuesday.
"I met with David Gill last week and he did not give me any of the assurances I was seeking about the future squad," Rooney said Wednesday in a statement. "I then told him that I would not be signing a new contract. I was interested to hear what Sir Alex had to say yesterday and surprised by some of it.
"It is absolutely true, as he said, that my agent and I have had a number of meetings with the club about a new contract. During those meetings in August, I asked for assurances about the continued ability of the club to attract the top players in the world."
Rooney has won three Premier League titles and the Champions League once since joining United in 2004 from boyhood club Everton for $40 million.
But United only won the League Cup last season after losing Cristiano Ronaldo and striker Carlos Tevez, without signing any similarly high-profile replacements - despite cash reserves of $258 million disclosed in accounts for the fiscal year ending June 30.
United's ability to compete for the world's top players is hampered by its debts, resulting from the 2005 leveraged takeover by the Glazer family.
Rooney has also seen neighbour Manchester City dramatically outspend United in the last two years since being bought by Abu Dhabi's Sheikh Mansour.
Like Ferguson did yesterday, Rooney downplayed talk about a rift between the two - calling the manager "a genius" - and said his decision was based solely on an ambition to win.
"I have never had anything but complete respect for MUFC. How could I not have done given its fantastic history and especially the last six years in which I have been lucky to play a part?" Rooney said. "For me its all about winning trophies - as the club has always done under Sir Alex. Because of that I think the questions I was asking were justified.
"Despite recent difficulties, I know I will always owe Sir Alex Ferguson a huge debt. He is a great manager and mentor who has helped and supported me from the day he signed me from Everton when I was only 18. For Manchester United's sake, I wish he could go on forever because he's a one off and a genius."
