The Financial Services Authority (FSA) wants the promotion of unregulated collective investment schemes (Ucis) and similar products to be restricted to "sophisticated investors" and people who have a high net worth, for whom the products are more likely to be suitable.
The Ucis retail market is worth about £2.5 billion in the UK and some 85,000 ordinary retail investors have direct holdings in investments, which can hold assets like traded life policy investments, fine wines, crops and timber.
Ucis are not subject to the same rules as regulated collective investment schemes, which have certain safeguards such as making sure risk is prudently spread.
While this means that those operating schemes have greater freedom to pursue new or unorthodox strategies, it also means investors are generally placing their cash at a higher risk than more mainstream investments.
The FSA said it has uncovered "high levels" of unsuitable advice being given and warned that ordinary investors are being exposed to significant potential for large losses on products which are clearly unsuited to most people.
It said examples of unsuitable advice it had found included pensioners being advised to invest all of their wealth in a single Ucis and a customer who was advised to borrow money to invest in Ucis and service the debt with withdrawals from their investment.
Under current rules, the investments can be promoted to ordinary investors if an adviser first assesses the product's suitability.
But under the watchdog's plans, firms will be prevented from marketing Ucis to ordinary retail customers, even in the context of financial advice.