The invention relates to a method of estimating a noise and interference covariance matrix and to a receiver. The method comprises: estimating an initial noise and interference covariance matrix on the basis of a received signal; reducing the impact of the background noise of the receiver from the initial noise and interference covariance matrix for obtaining a residual matrix; accepting the obtained residual matrix when the residual matrix is at least approximately positive semidefinite; modifying the obtained residual matrix such that the positive semidefinity of the residual matrix is achieved when the residual matrix is not at least approximately positive semidefinite; and adding the impact of background noise back to the residual matrix for estimating a final noise and interference covariance matrix.