The application of formal methods in the development of safety-critical embedded software is recommended in order to provide strong guarantees about the absence of software errors. In this context, WCET estimation tools constitute an important element to be formally verified. We present a formally verified WCET estimation tool, integrated to the formally verified CompCert C compiler. Our tool comes with a machine-checked proof which ensures that its WCET estimates are safe. Our tool operates over C programs and is composed of two main parts, a loop bound estimation and an Implicit Path Enumeration Technique (IPET)-based WCET calculation method. We evaluated the precision of the WCET estimates on a reference benchmark and obtained results which are competitive with state-of-the-art WCET estimation techniques.