The agency's chief executive, Sean Quinn, said the public and policy-makers did not appreciate tourism's importance.

"We feel very strongly that the true value of tourism is not appreciated so it's an objective of ours to raise it up the agenda for the public and politicians, " he said.

The billboard campaign, which begins tomorrow and will run for four weeks, concentrates on the fact that tourism generated 6bn for the economy last year and stresses that people should help maintain this by being welcoming to visitors and not littering.

Quinn said it was launching the campaign because "there are many challenges facing the development of tourism in Ireland and most of these are outside the tourism industry".

He said one of the major factors behind the campaign was market research carried out by the agency last year.

"A very high proportion of overseas visitors said litter was a bigger problem here than in their home countries."

Quinn said this did not mean that tourists were being put off Ireland; the research showed that tourists felt that "traditional warm Irish welcome" was still present.