It has been shown that potential business benefits could be achieved by assessing and improving the requirements engineering (RE) process. However, process assessment models such as CMMI and ISO9000 only cover RE shallowly. Tailored models such as REGPG and REPM, on the other hand, do not cover market-driven requirements engineering. Other attempts such as MDREPM covers market-driven requirements engineering, but correspondingly neglects bespoke requirements engineering. Moreover, the area itself has evolved so practices that once were cutting edge are now commonplace. In this article, we develop and evaluate a unified requirements engineering process maturity model (Uni-REPM) that can be used in a market-driven as well as a bespoke context. This model is based on REPM, but has evolved to reflect contemporary requirements engineering practices. Uni-REPM is primarily created based on a systematic literature review of market-driven requirements engineering practices and a literature review of bespoke practices. Based on the results, Uni-REPM is formulated. The objective of Uni-REPM is twofold. Firstly, it is expected to be applicable for assessing the maturity of RE processes in various scenarios where an organisation would use different development approaches. Secondly, it instructs practitioners about which RE practices to perform and their expected benefits. As an assessment instrument, Uni-REPM provides a simple and low-cost solution for practitioners to identify the status of their RE process. As a guidance tool, Uni-REPM lessens the gap between theoretical and practical worlds by transferring the available RE technologies from research to industry practice.