The companies were forced to put out a statement disclosing their talks on Tuesday following movements in UBM's share price.
Under Takeover Panel rules, Informa has until 1700 GMT on February 13 to make an offer or walk away for at least six months.
A proposed deal, revealed jointly by the two companies early Wednesday morning, would see Informa chairman Derek Mapp take over as chairman of the combined company, with Stephen Carter as its new CEO.
FTSE 100 listed Informa PLC (LON:INF) today launched an agreed takeover of its mid cap peer UBM PLC (LON:UBM) to create a leading B2B information services group valued at around £10bn, nearly a decade after UBM itself made a failed attempt to takeover Informa.
Big event: Informa and UBM to merge, creating a £9bn B2B information services group
The new group's board of directors would consist of seven non-exec directors from Informa and three from UBM, and the combined company would be 65.5 percent owned by Informa shareholders, and 34.5 percent by UBM shareholders.
The proposed deal would create a leading B2B Information Services Group and address an industry trend towards increased operating scaleand industry specialisation. In exchange for each UBM share, UBM stockholders would receive 1.083 Informa shares and £1.63 ($2.26).
UBM separately said the proposed combination is expected to result in significant synergies for the combined company, which be headed by Carter as chief executive.
It carries out activities ranging from academic publishing to hosting global exhibitions, knowledge and networking conferences and... The merger would be achieved through an acquisition of the entire issued and to-be-issued share capital of UBM by Informa, for shares and cash consideration.