Update 15.45: Gartmore's share price has jumped further in afternoon trading and is now up 11.7% to 101.1p, whilst Henderson had advanced nearly 10% to 152p.
Earlier, the market has responded positively to news Henderson has made a £335m bid for Gartmore, pushing its share price up 5.4% in early trading to 95.5p.
After weeks of negotiations, Henderson and Gartmore have finally reached a deal which values Gartmore's shares at 92.1p, a slight premium to its closing price of 90.6p on Tuesday.
This means Gartmore shareholders will be offered two Henderson shares for every three Gartmore shares they hold.
The offer is less than suggested by reports before Christmas which said Henderson would offer around 104.8p per share for Gartmore, valuing the beleaguered fund manager at around £344m.
Gartmore's shares soared today on the news a full acquisition should now take place in the next few months, while Henderson's share price also rose by 7.81% to 149p.
Gartmore has been rocked by a series of high-profile staff departures, which has caused its shares to collapse by more than 50% from their listing price of 220p in December 2009.
The deal with Henderson is subject to shareholder approval but boards for both groups have recommended shareholders accept the offer and the majority of Gartmore's fund managers have agreed to the terms.
The managers, which represent 84% of Gartmore's AUM, that have already agreed to join Henderson are Charlie Awdry, Chris Palmer (both emerging markets), John Bennett (European equities), Chris Burvill (Cautious Managed), Rob Giles and Adam McConkey (both UK small cap).
Also joining are Tony Lanning (multi-manager), Luke Newman, Ben Wallace (both UK long/short equities), Simon Peters (long/short financials), Neil Rogan (global equities) and John Stewart (Japan long/short).
The combined house will have £78m in assets under management, including £6bn in absolute return products.
Completion of the acquisition is expected to take place in the next three months with consolidation of the fund range to be finished by the summer.
In a stock exchange announcement, Andrew Formica, chief executive of Henderson, says: "The acquisition of Gartmore is a great opportunity for Henderson. Gartmore has a highly complimentary strategy and stable of products to that of Henderson.
"Its recent travails should not overshadow the fact that Gartmore is one of the best known managers in UK asset management and its assets are performing well.
"By bringing across fund managers and integrating the business on to our own platforms we will be able to enhance margins significantly. We will also better serve both sets of clients by expanding our product range, for instance in absolute return."
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