The 21-year-old Portuguese talent is attracting significant interest from several Premier League clubs, with West Ham now ready to make a concrete move after closely monitoring his progress.
The Hammers, who are weighing up a number of midfield targets this transfer window, view Fernandes as a strong addition to their squad as they aim to strengthen ahead of the new campaign.
Southampton, relegated from the Premier League last season, are resigned to losing Fernandes but have made it clear they will not let him leave on the cheap.
The south coast club are holding out for a fee of around 30 million pounds, confident that his quality and potential will draw competitive offers.
Leeds United have also tracked Fernandes and rate him highly, believing he has the ability to become a top-level midfielder.
However, their interest remains tentative as they are prioritising attacking reinforcements and would need to sell a midfielder before moving for him. Financial constraints at this stage of the transfer window mean they are unlikely to challenge West Ham's bid.
With Fernandes expected to return to the Premier League next season regardless of his next destination, the coming weeks could prove decisive in determining where he will continue his career.
Bruno Fernandes and Wilson Isidor's late goals secured dramatic wins for Manchester United and Sunderland, while Jack Grealish's brilliance helped Everton edge Wolves in Saturday's Premier League action.
West Ham have officially announced the signing of Portuguese midfielder Mateus Fernandes from Southampton in a deal worth over 40 million pounds.
Werder Bremen have secured the services of Leeds United's versatile defender Isaac Schmidt on a season-long loan, with an option to buy.
Everton have confirmed the signing of highly rated winger Tyler Dibling from Southampton on a four-year deal running until the end of June 2029.
