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Wayne Rooney works out Tuesday with Manchester United. ((Paul Ellis/Getty Images))

Wayne Rooney blamed Manchester United's failure to spend heavily on strengthening its squad for his decision to refuse to sign a new contract with the 18-time English champions.

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 Sir Alex Ferguson made Rooney's decision publicly 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 US$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 $265 million Cdn 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 of more than $1.2 billion Cdn, 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 on Tuesday, 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."

As speculation mounted as to who among Europe's heavyweight sides would launch a bid for the England international, City manager Roberto Mancini insisted that his club had not been in contact with Rooney.

"We must think about our season and our players -- that's what's important at this moment," Mancini said. "In the summer, if there is a good player that could change something for us, we can talk about this.

"Rooney is a great player but I think he'll stay at United."

A move to City would see Rooney link up again with Tevez, who left United in 2009 after his contract ran out.

Real Madrid is another potential destination for Rooney, but coach Jose Mourinho feels that he will end up remaining at Old Trafford.

"If at the end of the day Man United decide that he is to leave, then give me a call," Mourinho said.

If Rooney was to favour a move to Madrid, he would link up again with Cristiano Ronaldo, who struck up a superb partnership with the England striker at Old Trafford before leaving for the Spanish giants for 80 million pounds in June 2009.

Chelsea was the first European power to remark on Rooney, its manager Carlo Ancelotti saying on Monday that his club would be keen to move for Rooney if the striker was on the market.

Spanish champion and three-time European Cup winner Barcelona was also widely reported as a potential bidder for Rooney.

"He is one of the greatest players in the world as there are very few players with his qualities," Barcelona coach Pep Guardiola was quoted as telling The Sun newspaper. "He's a player I very much admire."

But Rooney's form this season has fallen woefully short of the standards he set himself last term, when he scored 34 goals in all competitions to establish himself as one of the world's top players.

He has scored only once for United this season -- a penalty against West Ham -- and faces a spell on the sidelines after injuring his ankle in training on Tuesday.