Thomas Cook Chief Executive Harriet Green has stepped down in a surprise move that wiped off around a fifth of the market value from the U.K.-based travel operator.

During Green's two-year tenure, Thomas Cook was transformed from failing tour operator to solid company. She cut costs — closing shops, reducing its air fleet and selling hotels. The company is valued at just under 2 billion pounds ($3.2 billion), compared to just 148 million pounds in July 2012.

She said Wednesday she had intended to go once her work was complete. Green says "that time is now."

Green will be replaced by chief operating officer Peter Fankhauser, who has been with Thomas Cook for 13 years.

Thomas Cook's share price fell nearly 20 percent in early trading to 111 pence.