The UK brewer's board said it had "no hesitation" in slamming the door on the new bid, which would value the group at £7.3bn - up from £6.8bn in the initial offer.
S&N's hardline stance will increase the tension between it and its would-be suitors, and is likely an attempt to put the ball back in Carlsberg and Heineken's court.
Sir Brian Stewart, chairman of S&N, said: "Carlsberg and Heineken's marginally increased proposal continues their attempt to get S&N's unique portfolio of businesses on the cheap. The Board is highly confident in the actions being taken to maximise shareholder value, and strongly urges shareholders to take no action."
Under the terms of the bid, Heineken would take S&N’s business in the UK, including John Smiths, Kronenbourg 1664 and Foster’s brands. Carlsberg would take the BBH venture in Eastern Europe and S&N’s operations in France.
Chief executive John Dunsmore said S&N "particularly objected to Carlsberg's refusal to allow agreed information about (joint venture) BBH's prospects to be released".
S&N has filed for arbitration proceedings against Carlsberg, accusing the group of breaking conditions in the two brewers’ successful joint venture in Eastern Europe, BBH.
S&N is due to release a trading statement next week, which is expected to echo reports from other brewers of a tough summer for beer in the UK. The group referred to “challenging conditions” in Western Europe in a release last week.