  Recent experiments have shown the possibility to tune the electron transport
properties of metallic nanosized superconductors through a gate voltage. These
results renewed the longstanding debate on the interaction between intense
electrostatic fields and superconductivity. Indeed, different works suggested
competing mechanisms as the cause of the effect: unconventional electric
field-effect or quasiparticle injection. By realizing ionic-gated Josephson
field-effect nanotransistors (IJoFETs), we provide the conclusive evidence of
electrostatic field-driven control of the supercurrent in metallic nanosized
superconductors. Our Nb IJoFETs show bipolar giant suppression of the
superconducting critical current up to $\sim45\%$ with negligible variation of
both the critical temperature and the normal-state resistance, in a setup where
both overheating and charge injection are impossible. The microscopic
explanation of these results calls upon a novel theory able to describe the
non-trivial interaction of static electric fields with conventional
superconductivity.
