A new reorthogonalized block classical Gram-Schmidt algorithm is proposed that factorizes a full column rank matrix A into A = QR where Q is left orthogonal (has orthonormal columns) and R is upper triangular and nonsingular.With appropriate assumptions on the diagonal blocks of R, the algorithm, when implemented in floating point arithmetic with machine unit ε M , produces Q and R such thatThe resulting bounds also improve a previous bound by Giraud et al. [Num. Math., 101(1):87-100, 2005] on the CGS2 algorithm originally developed by Abdelmalek [BIT, 11(4):354-367, 1971].