Manchester United will be virtually powerless to prevent Cristiano Ronaldo from leaving the club if he has his heart set on a move to Real Madrid, football agents and employment lawyers said yesterday.

He could follow the path of players who want to wriggle out of a contract by adopting a well-worn strategy, Barry Silkman, the football agent, said. That involves grinding the club down with transfer requests and leaks to the press until they buckle.

“You would just keep knocking on the door and drive them mad,” Silkman said. “You tell them you are not happy. You'd get bits in the press. You just basically have to drive them mad by keeping on knocking on the manager's door and phoning them up.

“The agent would phone up the manager and say that his player really didn't want to be at the club any more and the player would keep going to the manager and say he wanted a move. The manager knows that if a player is not happy you can only do so much to keep them.”

So short of selling Ronaldo, United will be left with the option of either fielding an unmotivated player in the first team or continuing to pay him £120,000 a week to stew in the reserves on a period of gardening leave. But even that has its legal pitfalls, James Davies, one of the country's leading employment lawyers, said.

Davies, joint head of the employment and incentives department at Lewis Silkin, the London-based law firm, said: “Basic employment law provision would be that if he were to walk out on his contract, he would be in breach and Manchester United could try to hold him to it by forcing him to honour it.

“They could say that they require him to play, or threaten to put him on gardening leave, by sticking him in the reserves. The problem with that is that you're paying the player a fortune and if they don't want to stay, the pressure is strong to let him go because you will get millions of pounds and you won't have to pay his wage.

“If they did want to stick him in the reserves, the question of how long you can put an employee on gardening leave for is not clear, legally. Ronaldo could say that his skills will atrophy if he doesn't get to play at the top level, or that he has a right to play, so he could sue. That is a tough point of employment law. At the end of the day, these situations usually result in the player going.”

If Ronaldo were to refuse to turn out for Manchester United, the club could try to sue him for losses they suffered as a result of his actions, Davies said. But it is not clear how they would proceed. He added: “How do you measure that? Could you persuade a court that if a particular player had played, the club would have won a particular game or thousands more paying spectators would have turned up? How can you measure the loss?

“Deals are always done because nobody wins by seeking to enforce the contract, because of the vast sums of money at stake and the diminishing returns on the asset - the player. In practice, it is highly likely he'll go.”

In the past, some players desperate for a transfer have resorted to more unorthodox means of protest. Pierre van Hooijdonk, the former Holland forward, went on strike before the beginning of the 1998 season, refusing to return to Nottingham Forest. When he eventually saw sense and came back to the club, he was shunned by his team-mates. Chelsea claimed that William Gallas threatened to score an own goal if he was prevented from leaving the club, in 2006.

Silkman said: “I think Cristiano is too much of a professional to try to go on strike or anything like that. You've got to remember that no one has come out with cast-iron proof that he wants a move. But if a player does want out, the key is just to drive the club mad until they have to sell him.”

Bid for freedom

A football agent's guide to getting out of your contract

i) Ask for a meeting with the manager and say that you want to move

ii) Ask your agent to ring the manager to tell him that you want to move

iii) Leak stories to the newspapers, saying that you want to move

iv) Repeat the above three steps until the club agrees to sell

v) At no stage should you threaten to go on strike or score an own goal

Source: Barry Silkman, former Manchester City player, now an agent