Nanocellulose papers offer high tensile strength and modulus but suffer from drawbacks such as their brittle nature. We show that mercerisation of cellulose nanopapers in strong alkaline media for 2 min to 24 h results in the (partial) transformation of native cellulose I into the more ductile cellulose II allomorph. The strain to failure of mercerised nanopapers tripled compared to the original nanopapers while retaining their tensile strength in excess of 100 MPa at the expense of a slight drop in modulus resulting in a significant increase in toughness (total work of fracture). An additional advantage of mercerisation is a reduction in porosity of the nanopapers and increased transparency.