TUI AG fell 8.27 percent in early trading, while Thomas Cook shed seven percent in a market also unsettled by the Greek crisis.
The declines were more severe than the overall 2.14 percent slide of benchmark FTSE 100 index.
Stock markets elsewhere in Europe suffered even worse drops Monday, with Paris' CAC 40 shrinking 4.32 percent, Frankfurt's DAX 30 losing 4.26 percent and Milan and Madrid bourses shedding 4.35 percent.
The dive of TUI and Thomas Cook in London trading reflected investor concern about tourism activity following Friday's attack at a Tunisian vacation destination that appeared to target primarily Western tourists.
At least 15 Britons were killed, and the British government has warned that death toll could continue to rise.
In updated travel advice Sunday, the Foreign Office urged those who remained in Tunisia to be vigilant and said there was a risk of further incidents.
About 20,000 Britons were on package holidays at the time of the attack, and travel companies have laid on flights to repatriate those who want to come home.