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Real-time services over best-effort networks, such as teleconferencing over unspecified bit rate (UBR) class asynchronous transfer mode (ATM) networks or the Internet, are expected to be in high demand in the near future as such services become more cost effective. However, cell loss caused by buffer overflow at switches is a serious problem in this service class, and is responsible for degradation of the quality of service (QoS). A promising technology for improving the QoS of best-effort networks is the recovery of lost cells by forward error correction (FEC). Conventional FEC methods utilize a FEC matrix with fixed dimensions and which is usually designed to cope with the worst case of cell loss. In this paper, a new method that provides dynamic recovery capacity to deal with the current cell loss ratio through the use of a variable-dimension FEC matrix is proposed. The scheme also takes into account the coding method of the matrix to provide data prioritization. Numerical results show that the cell loss ratio and transmission efficiency of the proposed method are superior to those of conventional methods.