Downing Street officials say Britain’s secrets weren’t at risk despite a photograph showing Prime Minister David Cameron’s official red box of papers left in public view on a train table.

A front-page photograph Monday in the Daily Mirror showed the famous, battered red box marked “Prime Minister” apparently unattended while Mr. Cameron left his seat on a weekend train to Scotland.

The newspaper said other passengers could have rifled through the box and studied secret plans for military options in Syria or details about disagreements with Russia over policy toward Syria.

A Downing Street statement, however, indicated that Mr. Cameron’s security team was in place and the box was not unattended.

In the past, British government-owned laptops containing sensitive information have been left on public transport, prompting tighter rules on security.

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