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Mateus Fernandes joined Southampton last summer on a five-year deal
Adam Blackmore
BBC Radio Solent sports editor
West Ham have agreed a deal in excess of £40m to sign Southampton midfielder Mateus Fernandes.
The 21-year-old has been granted permission to travel for a medical in London.
Fernandes would become West Ham's third-biggest signing, behind Sebastian Haller (£45m) and Lucas Paqueta (£51m).
The move will be a boost for under-pressure Hammers manager Graham Potter, whose side have lost their first three matches of the season in all competitions.
Southampton look set to make a substantial profit on the £15m they paid to sign Fernandes from Sporting last summer.
The Portugal Under-21 international has played 46 times for Saints, scoring four goals and providing seven assists.
West Ham are also close to completing the signing of Monaco midfielder Soungoutou Magassa, 21, for a fee of about £17.3m. The France Under-21 international is expected to have a medical on Thursday.