Chairing his first meeting of the cabinet, Cameron told senior ministers from his Conservative party and their coalition partners, the Liberal Democrats, that the distraction of mobiles would not be welcome.
The new team faces the daunting task of implementing public spending cuts to reduce a record budget deficit, while ensuring Britain stays on the road to recovery following a deep recession.
Politicians have come to rely on mobile technology such as BlackBerrys so much that parliamentary authorities now allow their use in the House of Commons main debating chamber, albeit silently.
Newly-installed Justice Secretary Kenneth Clarke became the first to break the rule, drawing a rebuke from Cameron at the inaugural meeting of top ministers.
To laughter from the rest of the cabinet, the prime minister ordered him to cut short a telephone conversation so he could get down to the business of running the country.
Cameron has not spared himself from the ban, his spokeswoman said.
The new PM brought his cabinet together for the first time on Thursday, after forming a coalition government earlier in the week by striking a deal with the Lib Dems following polls on May 6 that failed to produce a clear winner.
