ENIC, whose managing director Daniel Levy is a Spurs season-ticket holder, revealed on Wednesday that they were holding discussions with Sir Alan. Levy, 38, will take up Sugar's seat on the board and become a non-executive director.
The outgoing chairman, who will also step down as a Spurs director said: "This is an exciting development for Tottenham.
"ENIC has a wealth of experience, both of European Club football and the exploitation of media and other commercial rights.
"I believe that ENIC is committed to the long term prosperity of the club and has the right business skills to take it forward from here."
ENIC, with both gaming and media business interests, already hold a 20.2 per cent stake in Glasgow Rangers as well as shares in Vicenza, FC Basle, Slavia Prague and AEK Athens.
Sir Alan will step down on completion of the deal, which is expected to take place in February next year.
The agreement sees ENIC increase its holding in the club from just under 3 per cent to 29.9 per cent after snapping up 27.4 million shares. The company has also been granted the option of buying a further 13.2 per cent stake in the club at a later date.
Levy said of the agreement: "We believe the long term profitability and capital value of Tottenham can be significantly enhanced through further exploitation of media and sporting rights associated with the club.
"Tottenham is a major British football club with a prominent sporting heritage. We are firmly committed to improving Tottenham's commercial viability as one of this country's top football clubs.
"We believe it can regain its position as a significant contender in both domestic and European competitions."
Sir Alan Sugar will stand down as Tottenham chairman after agreeing to sell part of his stake in the club to international sports entertainment business ENIC - English National Investment Company.