The teenager was taken to hospital after officers were called at 0840 hrs to an address in Orpington, south London, a police spokeswoman said.
She said the girl's condition was described as ''life-threatening''. No more details were immediately available.
Concern over levels of knife crime in Britain have prompted the government to double the maximum sentence for unlawfully carrying a knife to four years.
A national knives amnesty in the summer netted more than 100,000 knives during its five-week span, including machetes, meat cleavers and axes.
REUTERS SHB RN1931