By Manchester correspondent Alex Stevens

Anthony Martial has finalised an astonishing move to Manchester United worth £43.8million, Football Insider sources understand.

The 19-year-old striker arrived at United’s Carrington headquarters on Monday to seal one of the most sensational transfers of the summer window.

Sources have told Football Insider that the full cost of the deal, including add-ons, is £43.8million, to make Martial the world’s most expensive teenage signing ever.

Football Insider revealed earlier on Monday that Martial had agreed the terms of a United contract and been given permission by Monaco and the French Football Federation to fly to Manchester to complete a transfer to Old Trafford.

United opened talks with Monaco over a deal for Martial in a bid to solve their striker crisis. They made their move after the Ligue 1 club were eliminated from the Champions League last week.

United have been desperate to add more firepower to their attack, which has been led singlehandedly by Wayne Rooney in the opening month of the season.

Louis van Gaal sanctioned the late swoop for Martial, a France Under-21 international who has scored 15 goals in 69 matches for Monaco.

The youngster has only been a first-team regular at Monaco for one season but is regarded by United as a versatile forward of immense potential.

The Martial deal is United’s biggest single purchase of the summer, following the captures of Bastian Schweinsteiger, Morgan Schneiderlin, Matteo Darmian, Memphis Depay and Sergio Romero.

It comes as the club’s attempt to sign Sadio Mane from Southampton in a £20million deal has stalled, while other more optimistic targets like Thomas Muller and Gareth Bale have failed to reach fruition.

Van Gaal wants to bring more pacy and mobility to his front line, which is over-reliant on Rooney as Bayer Leverkusen-bound Javier Hernandez follows earlier summer departures Robin van Persie and Radamel Falcao out of the Stamford Bridge exit door.

Hernandez has agreed a £12million move to Leverkusen, after the Mexican was told by the manager that he can find a new club.

Van Gaal has spoken warmly of Rooney’s significance to the United cause but accepts that the 29-year-old needs support.

England’s talisman failed to score for a 10th consecutive Premier League match as United lost 2-1 at Swansea on Sunday, their first defeat of the season.