OVERSEAS exchange programmes for young people are being threatened by lack of financial support from the Government and commercial organisations. Youth welfare groups that organise such programmes complain that funding disappointments have led to several ventures falling through after careful planning, while others have had to be shelved or re-scheduled. The welfare groups are calling for more generous donations and sponsorship, especially from the industrial and commercial sectors. Failure to come up with such funding would mean ''local young people will not have chances to go overseas and have their horizons broadened''. ''The trips are all educational in purpose,'' explained Vivian Lau Man, a youth exchange officer of the Hong Kong Federation of Youth Groups, which offers several youth overseas study tours annually. ''We can't charge participants much because they are young and have low earning ability. We don't want to see any teenager deprived of the right to take part just because of a financial problem. ''But it is getting difficult to find sponsorship. There are so many voluntary groups looking for sponsors for their programmes. The competition is keen.'' Last year, the federation organised three tours to Germany, Canada and Inner Mongolia in mainland China, involving over 50 young ''ambassadors''. Participants, who were chosen for their out-going personality, language ability, and academic performance, contributed part of the expenses or enjoyed an all-expenses paid trip. The ambassadors to Germany, for instance, each paid only $4,000 for the 24-day trip. Other expenses covering travel, food and accommodation, were sponsored by Lufthansa German Airlines and the Sir Robert Black Trust Fund. Ms Lau said the federation, like other organisations, did not reserve a separate budget for youth exchange programmes. However, these programmes were usually costly and needed sponsorship. Looking for financial backing was not easy, she said. On average, about 40 letters to commercial industries seeking sponsorship for a study tour yielded only one or two positive replies. ''Because of the uncertainty, long-term planning is difficult. I think the public should support the programmes, especially the business community. Young people are our future. The more they study now, the more they will contribute to the industry in thefuture.'' Kwong Kam-fai, spokesman for the Boys' and Girls' Clubs Association of Hong Kong, said not only commercial firms, but also some Government departments were reluctant or unable to sponsor youth programmes. In the meantime, Chan Kam-cheung, deputy executive director of the Hong Kong Federation of Youth Groups, said they were preparing to form a Fund for Youth Development with the ''ultimate aim of setting up long term and stable resources to support the international exchange programme''.