He will join the Teessiders for the third time in his career on 5 August, subject to passing a medical and being granted a work permit.

Boro have been chasing Juninho, 29, since the end of the World Cup last month but had initially been unable to agree personal terms.

But now the popular midfielder is likely to make his third debut for the club in a pre-season friendly against Alaves on 7 August after agreeing a four-year contract.

Juninho made a major impact in the north-east after initialy arriving in a �4.75m move from Sao Paolo in November 1995.

He helped Boro reach both the FA Cup and League Cup finals in 1997, but could not prevent the club from being relegated from the Premiership the same year.

The fans' favourite subsequently moved to Atletico in a �12m deal, only to return to Boro on loan in September 1999.

But he was not able to reproduce the inspirational form of his first spell after suffering a leg injury, and was farmed out to Brazilian sides Flamengo and Vasco da Gama.

A return to form in his native country saw Juninho recalled to the Brazil squad that secured a fifth World Cup crown in Japan and Korea last month.

His return to the Riverside brings to a conclusion lengthy negotiations conducted by Boro chief executive Keith Lamb with Juninho's agent and father, Osvaldo Giroldo.

The club agreed a �6m fee with Atletico several weeks ago, but could not agree personal terms when Giroldo flew to England.

But Lamb then upped the offer, making it clear it was the club's best and final package.

Following more talks, the two parties came to an agreement which will will see Juninho become Steve McClaren's fourth summer signing.

The Boro boss has already signed Italian striker Massimo Maccarone, made Franck Queudrue's loan move permanent and secured Cameroon international Geremi on a year's loan from Real Madrid.

McClaren travelled to Spain from the club's Austrian training camp to seal the transfer.

He said: "I'm absolutely delighted Juninho has agreed to join us. We have a World Cup winner who is as ambitious as we are.

"I have spoken to him a couple of times and he wants to help make Middlesbrough successful. He feels he has unfinished business."