Tough new visa rules have been introduced for travellers from countries caught up in the Arab Spring unrest.
Under the changes Syrian, Libyan and Egyptian citizens will have to apply for a visa if they are travelling through UK airports.
The transit visa regime, introduced in response to threats to national and border security, covers passengers who are only in the UK to change flights and never leave the airport.
The three countries, which have seen major upheaval following the Arab Spring wave of protests, have been added to the list of nations covered by the Direct Airside Transit Visa (DATV) requirements.
Home Secretary Theresa May told MPs: "We see this as a necessary measure to protect the security of our borders."
She added: "The situation in Syria continues to pose a serious concern to us and the wider international community.
"Libya and Egypt are emerging from a period of instability.
"We therefore assess that requiring nationals of these countries to obtain a visa for transit through the UK is both a sensible and proportionate response to the threat posed to the UK's national and border security."
Mrs May said that since the introduction of the DATV regime in 2003, a number of countries have been added to the list of those required to obtain a transit visa.
"This has been done as a direct response to emerging security and immigration threats to the UK, and we will continue to monitor risks and threats," she told the Commons in a written statement.
"The implementation of the DATV regime allows us to run comprehensive checks on those transiting the UK, and prevents high harm individuals from travelling here.
"Since the original introduction of the DATV there has been a noticeable fall in transit passengers destroying their travel documents before claiming asylum.
"The visa process means that we are able to collect pre-travel information as part of the application process which makes identification and checks more robust."