The surprise development means the end of Emap, which began life in 1947 as East Midlands Allied Press.
On December 7, it agreed to sell its consumer magazine and radio divisions to German publishing company H Bauer for £1.14bn.
The Bauer deal is conditional on the approval of Emap shareholders at a meeting in January and today's B2B deal is dependent on the other deal going through.
Emap said the two deals will result in a return to shareholders of 931p a share.
The company's share price rocketed 20% from yesterday's close to 915p as of mid-afternoon today.
Guardian Media Group and Apax saw off competition from two private equity pairings, Candover and Cinven, and Providence and Permira.
Carolyn McCall, chief executive of Guardian Media Group, said: "The acquisition meets our requirements in terms of diversifying GMG's media interests, in line with our commitment to guarantee the long-term financial security of the Guardian."
Irina Hammers of Apax said the pair plan to extend Emap's franchises internationally.Emap's business-to-business assets include Construction News, Drapers, Retail Week and the Cannes Advertising Festival.