The last Roman ruler to persecute Christians, Diocletian brought stability to the empire after the chaotic third century. In 301AD, he passed his edict on prices, an unsuccessful attempt to stop inflation by imposing maximum prices on common goods.
Most contentiously the rules are proposed to apply to the staff of European banks who work outside the EU, in financial centres including New York and Singapore.
Supporters of the cap say it will discourage bankers from pursuing the sort of high-risk deals that helped cause the financial crisis. Opponents point out that hedge funds, private equity companies and other financial firms are unaffected.
Mr Johnson said the rules would only harm Europe.
“Brussels cannot control the global market for banking talent. Brussels cannot set pay for bankers around the world,” he said.
“The most this measure can hope to achieve is a boost for Zurich and Singapore and New York at the expense of a struggling EU.” Mr Johnson added: “People will wonder why we stay in the EU if it persists in such transparently self-defeating policies.”
Britain had attempted to block the rules, but was outvoted. Ministers can now only argue about how the cap should be applied.
Mr Cameron said on Thursday that Britain would push for more flexibility in the cap.