Recently, Zhang and Luo proved that a certain semigroup L of order six is non-finitely based. The main aim of the present article is to generalize this result to a sufficient condition for the non-finite basis property of semigroups. It follows that the semigroup L is inherently non-finitely based relative to a certain class of semigroups. It is also shown that the variety var L generated by L contains a unique maximal subvariety that is non-finitely based. Consequently, the variety var L is not a limit variety.