Edge federation lets service providers in different administrative domains share resources and services, but existing solutions depend on pre-established agreements, manual operations, high overhead and trusted third parties. Using the ETSI MEC framework as the reference architecture, the article proposes a blockchain and smart-contract based MEC Federator that allows distributed MEC systems to negotiate and execute federation dynamically, securely and automatically. A proof of concept built on a private Ethereum network (Hyperledger Besu), with a large number of MEC systems and two consensus algorithms, shows that the whole federation lifecycle, from negotiation to deployment, completes in about 18 seconds in the baseline scenario and scales well when many MEC systems request federation at the same time.