The Prime Minister has dismissed a clamour from regions of the UK for a cut in air tax following the Government's intervention in Northern Ireland.
David Cameron said the province was a special case.
Since George Osborne announced a cut in air passenger duty (APD) for Northern Ireland last month there have been demands from MPs representing other regions for similar measures. One airline boss told the Belfast Telegraph the Government had "opened a can of worms".
Mr Cameron said: "I understand the arguments. We do want successful regional airports.
"I'm happy to look at what will help regional airports, but we have a national taxation system and that would be difficult to change.
"We have made an exception for Belfast and devolved air passenger duty for a very specific reason. They have a different situation, with a land border with the Republic and a choice of where to fly from."
The Government is currently consulting on the rates of APD charged in the UK.
Mr Osborne intervened in Northern Ireland amid pressure from Continental Airlines on the province's only transatlantic route.
Formally confirmed this week, the measure means the rate of APD for direct long-haul passengers departing from airports in Northern Ireland will be cut from November 1 to the short-haul rate, currently £12 in economy and £24 in business and first class.
During questions to the Northern Ireland Secretary this week, Paul Goggins, a Labour MP for Wythenshawe and Sale East in Greater Manchester, said the move created an "anomaly" whereby passengers from Belfast could pay less tax to New York than to Manchester.
Stephen Hepburn, MP for Jarrow in the North East, welcomed the announcement, saying "how encouraging it is to other parts of the UK that think that particular tax is unfair".
Niall Duffy, head of public affairs for Flybe, said: "As predicted, this has opened up a can of worms. I don't think any of us are surprised."