New Manchester United minority owner Sir Jim Ratcliffe has said he wants to break the dominance of Manchester City and Liverpool in the Premier League and believes he can do it in three years.

Ratcliffe's 27.7% stake was formally signed off on Tuesday, and speaking on Wednesday, the 71-year-old said he wants to knock both City and Liverpool "off their perch."

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It's a reference to a quote made by Sir Alex Ferguson when he took over as United manager in 1986 when he said he wanted to topple Liverpool, who dominated the English league in the 1970s and 80s.

"We have a lot to learn from our noisy neighbour and the other neighbour," said Ratcliffe.

"They are the enemy at the end of the day. There is nothing I would like better than to knock both of them off their perch. They have been in a good place for a while and there are things we can learn from both of them.

"They have sensible organisations, great people within the organisations, a good, driven and elite environment that they work in. I am very respectful of them but they are still the enemy."

City and Liverpool have combined to win the last six Premier League titles, while Pep Guardiola's side are hoping to win a record fourth championship in a row this season.

United, meanwhile, won the last of their 20 titles under Ferguson in 2013 and are sixth in this season's table, 13 points off Liverpool at the top.

But despite more than a decade of underachievement at Old Trafford, Ratcliffe believes three years is a reasonable timeframe to bring back success.

"The fans would run out of patience if it was a 10-year plan," he said.

"But it's certainly a three-year plan to get there. To think that we're going to be playing football as good as Manchester City played against Real Madrid last year by next season is not sensible.

"And if we give people false expectations, then they will get disappointed. So I think the key thing is our trajectory, so that people can see that we're making progress. Because it's not easy to turn Manchester United into the world's best football team."

Meanwhile, Ratcliffe, who was speaking in London, has branded Newcastle "silly" following negotiations to hire sporting director Dan Ashworth.

Newcastle want a compensation package of between 15 million ($18.9m) and 20m ($25.2m), with the 51-year-old executive also serving a lengthy period of gardening leave.

Ratcliffe has also left the door open for the possible return of Mason Greenwood, who is on loan at Getafe following criminal and club investigations into alleged abusive behavior.

"I can talk about the principle, I'm not going to talk about Mason," Ratcliffe said. "I'm familiar with it. The principle is the important one because we will have other issues going forward.

"We'll make a decision, yeah, correct. And we'll justify it one way or another.

"All I can do is talk about the principle of how we will approach it. Is he the right type of footballer? Is he a good person or not?"