Midfielder Ryan Gauld has completed his move to Sporting Lisbon from Dundee United for an undisclosed fee.

The 18-year-old has agreed a six-year contract with the Portuguese club, which includes a 60m euro buy-out clause.

Both clubs issued short statements confirming the deal for the Scotland U21 international.

"Everyone at the club wishes Ryan sincere best wishes in his future career," said United.

A series of eye-catching performances last season earned Aberdeen-born Gauld the nickname 'Mini-Messi' and he was short-listed for the PFA Scotland Young Player of the Year, which was won by team-mate Andy Robertson.

Having made his United debut in May 2012, his rise to prominence with Jackie McNamara's team led to an extended contract at Tannadice as soon as he turned 18.

He scored eight goals in 38 appearances in the last campaign as United finished fourth in the Premiership and runners-up in the Scottish Cup.

United boss McNamara won 33 caps for Scotland and has every faith that Gauld will be a key player on the international stage.

"He's going to be a massive star in our national team, there's no doubt about that," McNamara told BBC Scotland in December. "He will be one of the greats of our game."

Champions of Portugal on 18 occasions, Sporting finished second behind city rivals Benfica in the Primeira Liga last term.