William Carvalho
Carvalho scouted by both Arsenal and United. (Reuters)Reuters

Manchester United have reportedly been warned that they must pay £37m to secure Sporting Lisbon midfielder Wiliam Carvalho, as the Portuguese side are unwilling to negotiate a deal below his release clause.

The 21-year-old midfielder is one of the most sought-after midfielders around Europe, with United, Arsenal, Real Madrid, Juventus and Barcelona reportedly interesting in acquiring his services.

The Daily Mail recently claimed that United were leading the race for his signature, claiming that the Premier League holders had begun negotiations with Sporting trying to close a £29m deal before the summer, to avoid a price rise following the World Cup.

However, Portuguese newspaper A Bola reports that sources close to Sporting Lisbon assured that they are not willing to negotiate a deal below his £37m release clause.

Indeed, A Bola adds that Sporting are set to offer Carvalho a pay rise to persuade him to commit his future to his current club at least one more season.

The midfielder is proving to be a sensation in Portugal this season, being named the best player in the league by supporters during October and November and securing a place in the national team for the Brazil World Cup.

United's scouts have been spotted at the Alvalade Stadium on a number of occasions this season and they have reportedly passed excellent reports to Moyes, who is desperately looking to bolster his midfield.

Meanwhile, Carvalho recently fuelled the speculation about a summer move, praising both United and Real Madrid.

"It's nice being praised and to know there are some big clubs interested in me," Carvalho told reporters at the beginning of the month.

"It could be Manchester United or Real Madrid. It's just nice to have a great European club after me, but at the moment I'm just totally focused on Sporting and doing my best until the end of the season. At the end of the campaign, we will see what happens."