A series of tests are to be carried out to see how toxic gas would spread if it was used by terrorists in London.
Harmless, odourless gas will be released in the Marylebone area during the six week-long trial.
Scientists will then use specialist equipment to measure levels of the gas and determine how long it stays in the air and where it moves to.
Results of the Home Office tests, which start on Sunday, will be used to help improve public protection.
Pressurised canisters
The study is part of the government's chemical, biological, radiological and nuclear (CBRN) Science and Technology Programme.
It will involve non-toxic perfluorinated carbons being released into the atmosphere from small pressurised canisters.
The Home Office said: "The trials will improve our understanding of the movement of air-borne material in the urban environment and will enable enhancements in public protection to be developed."
Similar trials were carried out on the Tube network by the Department of Transport in March and April.
On two consecutive Sundays harmless sulphur hexafluoride was released inside the Jubilee Line's St John's Wood station in west London.
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