The German government - made up of Chancellor Angela Merkel's conservatives, their Bavarian sister party CSU and the Social Democrats (SPD) - is expected to present a far-reaching package of climate protection measures on Sept. 20.

The tax for domestic flights is now 7.40 euros (£6.60).

"We want to double this ticket tax for all domestic flights and triple it for short-haul domestic flights under 400 kilometres," said a party document that the board of Merkel's Christian Democrats (CDU) is due to review on Monday.

The document, called "Climate-friendly Germany - using innovations as we head into the future", showed that the party also wants to make long-distance rail travel cheaper by reducing the value-added tax on train tickets.

A spokeswoman for Germany's flagship airline, Lufthansa, said on Friday that air transport faced more international competition than other transport and such national measures could result in traffic shifting to regions where levies are smaller or non-existent.

"Such a high burden for domestic flights disproportionately affects German airlines and deprives them of money for modern airplanes and climate-friendly technologies," the spokeswoman said of the CDU proposal.

She added that domestic flights only accounted for 0.3% of Germany's overall carbon dioxide emissions and it was questionable whether such measures would have any tangible ecological impact.

The document also said a premium worth several thousand euros should be introduced to encourage people to scrap old, inefficient heating systems so that climate targets for buildings can be achieved.

(Additional reporting by Ilona Wissenbach in Frankfurt, writing by Michelle Martin; editing by Paul Carrel, Larry King)

By Markus Wacket