The government is to carry out tests to see how toxic substances would spread if used by terrorists on the Tube.
Emergency services have previously carried out tests on the Tube
A harmless chemical - sulphur hexafluoride - will be used to monitor airflows at north-west London's St John's Wood London Underground station.
Services will run as normal on the two Sundays chosen for the tests, 25 March and 1 April.
Scientists from the Ministry of Defence will be in charge of the tests, which they say pose no safety risks.
It follows trials on both the Tube and at mainline train stations of passenger screening, with people volunteering to take part.
The first trial of passenger screening technology took place at Paddington station in west London in January 2006 and was followed by further trials at Canary Wharf and Greenford Tube stations.
Trials also took place last August to test the practicalities of deploying portable vehicle access control barriers at major entry points at Waterloo and Victoria stations in London.
Announcing the St John's Wood tests, Transport Secretary Douglas Alexander said: "The purpose of the study is to gather data within a genuine rail environment.
"It is not a reaction to any threat increase or a measure to enhance security at this or any other station.
"All the data and feedback gained will merely help to inform future decisions."
Mr Alexander said the public understood providing airport-style security on open systems such as the rail and Tube networks was unworkable and that no single security measure was either foolproof or capable of mitigating every threat.