Shares in Misys fell by 18% in morning trading after the UK fintech vendor's proposed takeover by American counterpart FIS broke down.
US firm FIS approached Misys about a possible cash offer last month worth in the region of £1.4 billion and the pair entered discussion.
Last week a revised non-binding offer was tabled but Misys says it "materially undervalues" the company and that the board "unanimously decided to reject it and took the decision to withdraw from further discussions with FIS".
The suitor insists it will not increase its bid and has walked away, saying that it is "no longer considering making an offer".
In a statement, Misys says it "has confidence in the future opportunities", highlighting the strong full year results it posted last week, where it recorded a 12% rise in full year adjusted profits.
However, shares in the company have taken a battering in early morning trading, down 18%, or 64.8 pence, to 293.4 pence.