People from India, Pakistan, Bangladesh, Angola, Cameroon and Lebanon will need visas to travel through the UK under new British immigration controls.
Ministers want to bolster asylum controls
The new rules come into force from midnight on Wednesday as the Home Office tries to stop people exploiting the current rules to enter the UK illegally or make baseless asylum claims.
Until now, nationals from the six countries have been able to travel without a visa if they are only passing through Britain on the way to a third country.
The new rules mean they will have to apply for the £27 visa before travelling.
A Home Office spokeswoman told BBC News Online: "In theory, you can claim asylum in the first country you reach after you have left your country of origin."
That meant asylum could be claimed by somebody stopping at Heathrow just for an hour-and-a-half to switch planes or refuel on their way to another country - something the government now wants to prevent.
The Home Office says it will try to minimise the inconvenience to genuine onward passengers.
People who have already booked their travel tickets will not need the visas if they travel before midnight on 20 October.
To show all visa regimes are kept under constant review, the government also says it is lifting transit visa regimes for Croatia, Libya and Slovakia.
Home Office Minister, Beverley Hughes, said: "We are responding to intelligence that some nationals of these countries are using transit visas to flout our immigration controls and either enter the UK illegally or make unfounded asylum applications.
"Those who do so have no right to be in the UK and we are committed to stopping them from travelling here in the first place.
"This year, we have tightened up visa regimes for 18 countries, and this is helping to reduce the number of asylum claims and illegal entrants."