The Council of Europe's Group of States against Corruption (GRECO) praised Switzerland's existing anti-graft laws but said the number of convictions was low relative to the number of investigations opened.
It said that although Swiss law punishes corrupt public officials, there had been no convictions so far in private bribery cases -- a fact the body called "a matter of concern".
The organisation also stressed that "the Swiss legal system is almost alone in Europe in not imposing -- at federal level and in nearly all cantons -- any rules on transparency of political funding."
"Political parties are not subject to any binding rules in this area or to any supervisory arrangements concerning their income and expenditure," it said, adding that this was also true for election campaigns and referendums.
The council said Switzerland, given its economic and financial weight and the many multinationals with headquarters there, appears "particularly exposed to the risks of bribery in the private sector and of foreign public officials".
Part of the problem was that Swiss courts only deal with private corruption cases once they receive a complaint from a company, individual or group, said the organisation, recommending that this constraint be lifted.
GRECO, set up in 1999 by the Council of Europe, comprises 48 European states and the United States.