You’ve not told us your nationality, so we can’t say for certain. Also, you haven’t said if you’re changing airports, or if you want to leave the airport at any point even if not.
The UK now requires nationals of quite a few countries to have a visa when transiting through the UK, including airside transit. That visa could be a UK transit visa, or it could be a visa for certain destination countries. The US is on that list, so even if you would normally need a transit visa for the UK, if you’re in transit on the way to/from the US and hold a valid visa for the US, you don’t need a UK visa to transit.
There’s a handy widget on the UKBA website which helps you work out if you’ll need a transit visa or not, based on your nationality, destination and transit type. They also provide quite a bit of information there.
For the “Transit without visa concession” bit, which allows you to transit through the UK to certain countries, see the Do I Need a Transit Visa page and then click to expand the “‘Transit without visa’ concession” section for details. Currently, if you have a valid US, Canadian, Australian or New Zealand visa and are transiting through the UK then you don’t need a visa.
You might also find this past question useful: Do I need a UK or Ireland transit visa when flying from the US to a Schengen country?
Depending on your unique situation, use this official resource for finding if you need a visa.
Anecdotal: I have heard of cases where the departure flights from the US forbid boarding due to the absence of required transit visa for layovers in London.
On the other hand, if you plan on stepping out of the airport, you will, of course, need a tourist visa, per your country of citizenship.
You haven’t mentioned what your nationality is or whether you’re a resident of any third country.
Having said that, in most countries (US being a notable exception), you can Transit Without Visa (TWOV) as long as the connection time is less than 24 hours. In your case thus you do not need a transit visa – as long as you remain at the airport.
There is a nifty widget to check if you need a transit visa or not.
This is from the UK Border Agency website:
If you are coming to the UK briefly as part of your journey to another
country, you will generally need to obtain a UK visa before you
travel.
- If you will arrive on a flight, remain in the arrival lounge of the airport without through immigration control, and then depart on
another flight from the same airport, you should apply for a direct
airside transit visa.If you will arrive in the UK, pass through immigration control and then leave the UK within 48 hours, you should apply for a visitor in
transit visa.If you will spend more than 48 hours in the UK, you must obtain another type of visitor visa before you travel. The Visiting the UK
section lists the types of visitor visa.In some circumstances you may be exempt from the need to obtain a visa
– this is called the ‘transit without visa concession’. The Transiting the UK section contains more information, and explains how to apply
for a transit visa.
India is not on the list of countries that need a transit visa for Ireland.
EDIT: I was just informed by a friend that if you hold a US visa you are except from a transit visa if the USA is your point of origin or destination. I checked the UK border agency site and saw no mention if it. If anyone can confirm this, that would be handy.
Credit:stackoverflow.com‘