Boston, don't drink the water!

Nearly 2 million residents of Greater Boston lost their supply of clean drinking water when a huge pipe abruptly burst yesterday, prompting authorities to declare a state of emergency and to impose a sweeping order for homeowners and businesses to boil the untreated water now flowing from their taps.

Governor Deval Patrick said residents in Boston and 29 other communities east of Weston should boil water for at least a minute before drinking it to avoid the risk of getting sick. He also asked bottled water companies and the National Guard to help make clean water available to residents in the affected communities.

The crisis began around 10 a.m. yesterday when a 10-foot-wide pipe in Weston sprang a leak, which worsened throughout the afternoon and eventually cut off Greater Boston from the Quabbin Reservoir, where most of its water supply is stored.

The Massachusetts Water Resources Authority said it could continue supplying water by activating a backup system that began drawing water last night from the Sudbury Reservoir, and can also tap into the Weston and Spot Pond reservoirs if necessary. The backup water, which one official compared with “untreated pond water,’’ can be used for bathing and flushing toilets, but not for drinking or cooking.

Authorities said they were attempting to set up mobile units to chlorinate the backup water supply, but they cautioned that even so, the water from the backup system would not meet federal drinking water standards.

“This is everyone’s worst nightmare in the water industry,’’ said Frederick A. Laskey, executive director of the Massachusetts Water Resources Authority.