A method is disclosed for increasing strength properties, preferably burst strength and SCT strength, of a paper or board product. The paper or board product is manufactured from a fibrous web produced by a multilayer headbox, where an aqueous layer is formed between at least a first and a second fibre layer formed from fibrous stock suspension(s), and where feed water for the aqueous layer includes at least one cationic polymer. The method of the invention includes adding an anionic additive selected from a group comprising anionic synthetic organic polymers, anionic polysaccharides, and any of their combinations to the feed water before formation of the aqueous layer.