Old Tiffinians secured a spot in the RFU’s London leagues with a 37-24 win over Old Whitgiftians on Saturday.
It meant the East Molesey-based Tiffin School old boys secured the Surrey Division One title with three games still to play.
Club captain James Fentiman described the triumph as fantastic and hailed the club’s forwards, who have dominated all season.
“We have got some amazing centres, but our forwards have definitely laid the foundations for our success,” he said.
“There were a few sore heads on Sunday and no doubt there will be a few more celebrations to come.”
He added: “We are in the London leagues for the first time in our history, which is amazing to think we once asked to be demoted to Surrey Division Three.”
In the same league, Kingston won 32-7 at third-placed Old Wimbledonians to move off the foot of the table with two games to play.
Kingston remain in the dropzone, but, with six points separating the bottom four teams, survival remains in their own hands.
Tiffs go to Old Freemens on Saturday.