Context : Magnetic fields are thought to be one of the possible mechanisms responsible for shaping the generally spherical outflow of evolved stars into often aspherical planetary nebulae .
However , direct measurements of magnetic fields during the transition to the planetary nebula phase are rare .

Aims : The aim of this project is to expand the number of magnetic field measurements of stars in the ( proto- ) planetary nebula phase and find if the magnetic field strength is sufficient to affect the stellar outflow .

Methods : We used Very Long Baseline Array observations to measure the circular polarization due to the Zeeman splitting of 22 GHz H _ { 2 } O masers in the envelope of the proto-planetary nebula candidate star IRAS 19296+2227 and the planetary nebula K3-35 .

Results : A strong magnetic field of B _ { || } = -135 \pm 28 is detected in the H _ { 2 } O maser region of the proto-planetary nebula candidate IRAS 19296+2227 .
The H _ { 2 } O masers of K3-35 are too weak to detect circular polarization although we do present the measurements of weak linear polarization in those masers .

Conclusions : The field measured in the masers of IRAS 19296+2227 is dynamically important and , if it is representative of the large scale field , is an important factor in driving the stellar mass loss and shaping the stellar outflow .