If you get an urgent message on your cell phone from President Obama later this year, it’s not a prank.

Under a new emergency notification system being announced tomorrow by Mayor Bloomberg and federal officials, anyone carrying an “enabled” mobile device within range of a cell phone tower would be alerted what to do in case of emergency.

“If there’s a terrorist bomb set to off in Times Square, you’d be able to tell everyone in the vicinity to get out of there,” explained one official.

The service is scheduled to be available in New York City and Washington, D.C. by the end of the year and in the rest of the country by mid-2012 as part of a law passed by Congress five years ago.

But to receive the alerts, cell phones and other wireless devices will need a special chip that is now only in a few models of mobile phones. Cell phone companies, however, have agreed to add the chip to new phones and wireless devices rolling out by 2012.

The alerts would be very brief — no more than 90 characters.

Officials said they would fall into three categories: a warning issued directly by the President; a message involving imminent threats to safety and amber alerts about missing kids.

Cell phone companies could allow subscribers to block all but the presidential messages.

New York City already has a method to contact residents in a hurry via cellphones — but only for those who sign up in advance.

Since its launch in May 2009, “Notify NYC” has attracted 67,000 subscribers and 17,000 Twitter followers.

While a ground-breaking initiative, that still represents a mere fraction of the city’s population.

Bloomberg described the new system, called Personal Localized Alerting Network or PLAN, as a “quantum leap forward.”

“Given the kinds of threats made against New York City at the World Trade Center, Times Square and other places popular with visitors and tourists, we’ll be even safer when authorities can broadcast warnings to everyone in a geographic area regardless of where they came from or bought their phones,” he said.

The announcement will be made tomorrow at the World Trade Center site with FCC chairman Julius Genachowski, Federal Emergency Management Agency Administrator W. Craig Fugate and top executives of the participating cell carriers, AT&T, Spring, T-Mobile and Verizon.