Brazilian midfielder Casemiro has joined Argentine star Lionel Messi to play for Inter Miami, the latter announced Wednesday, but the Major League Soccer (MLS) immediately said it is investigating allegations that the club violated negotiating rules in the deal.

Casemiro, who has 91 international caps for Brazil and won the UEFA Champions League five times with Real Madrid, became a free agent after Manchester United decided not to renew his contract following a mixed four-year spell with the Premier League giant.

The 34-year-old midfielder chose to join Messi and fellow Argentine Rodrigo De Paul at Inter Miami, the club co-owned by Englishman David Beckham.

Jorge Mas, managing owner of Inter Miami, said: 'Casemiro's arrival reflects the vision and ambition that define Inter Miami. We are never satisfied. We always seek to grow, improve and raise our standards every season.'

He added: 'We have built a global club with a vision to become not only the best club in the United States, but also a global reference.'

But moments after the announcement, MLS said in a statement: 'We are reviewing allegations regarding a violation of the league's negotiating rules by Inter Miami CF.'

The investigation concerns the 'discovery rights' in place in MLS, which clubs must obtain before beginning negotiations to sign certain prominent players.

The LA Galaxy held the discovery rights for Casemiro, and it is understood they will receive compensation from Inter Miami.

The statement said: 'The league is gathering all relevant information and will refrain from further comment until the review is complete.'

It added: 'While Inter Miami and the LA Galaxy have reached a settlement regarding discovery priority for Casemiro, its terms will be announced after the conclusion of the investigation into the alleged negotiating rule violation.'

While other clubs also boast prominent international stars, such as South Korean Son Heung-min (Los Angeles FC), the signing of Casemiro strengthens Inter Miami's status as the most star-studded club in MLS.

In December, Inter Miami won its first league title and moved into a completely new stadium in April.

But the club has previously faced issues related to player transfers.

In 2020, the transfer of Frenchman Blaise Matuidi, 2018 World Cup winner, from Juventus to Inter Miami prompted an MLS investigation that resulted in a $2 million fine for violating salary cap rules.

The league resumed activity last week after a mid-season break due to the World Cup, and hopes to capitalize on the football fever generated by the global finals that concluded Sunday with Spain winning the title at the expense of Messi and Argentina.

One of the league's promotional slogans after the World Cup read: 'Thank you, world. From here, we'll take it from here.'

Casemiro said in a club statement that he is motivated to 'win and continue to evolve' with Inter Miami, adding: 'The project the club presented to me, along with the effort everyone made to bring me here, means a lot to me.'

Beckham, for his part, said: 'He is a winner who has achieved so much in the game, and after an amazing career with Real Madrid and Manchester United, I am very happy that he has decided to make Miami his new home.'