Shares in the FTSE 250 firm crashed 151p to 269¼p as it slashed its target for annual cost savings from £100million to £75million.
Underlying profit will by squeezed by extra charges of about £10million relating to Brexit, IT security, regulatory and legal costs.
Chief John Phizackerley has been replaced by head of global broking Nicolas Breteau.
Chairman Rupert Robson, who is stepping down at the end of the year, said: “The evolving landscape is driving up costs across our industry. The acquisition of ICAP has given us greater scale to withstand this pressure.
“However, it has become clear that a change of leadership is required to execute our medium-term growth strategy and deliver the detail of the integration process.”
TP Icap also made Robin Stewart’s interim role as chief financial officer permanent.
“We believe that Nicolas and Robin make an excellent team to lead the company for the future,” said Mr Robson.