A mother in easternmost Canada pleaded guilty on Friday to murdering her only child in a twisted bid to save her relationship with a boyfriend, a court official said.

Penny Boudreau, 34, was sentenced to life in prison with no possibility of parole for 20 years for strangling her 12-year-old daughter Karissa after her boyfriend had given her an ultimatum: Karissa or him.

The Supreme Court of Nova Scotia heard she drove Karissa to a remote spot near the town of Bridgewater in January 2008 and told her to get out of the car.

They argued and Penny tackled Karissa to the ground, pinned her with her knees, wrapped twine around her neck and strangled her.

"Karissa said: 'Mommy, don't'," prosecutor Paul Scovil told the court, as relatives sobbed, according to local media.