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The most extreme ultraluminous X-ray sources (ULXs), with $L_\mathrm{X} > 5\times 10^{40}$ erg s$^{-1}$, are amongst the best candidates for hostingintermediate-mass black holes (IMBHs) in the haloes of galaxies. Jet radioemission is expected from a sub-Eddington accreting IMBH in the low/hard (radiobright) state. In a search for such IMBH jet radio emission, we have observedwith the Very Large Array (VLA) at 5 GHz a sample of seven extreme ULXs whoseX-ray properties indicate they are in the hard state. Assuming they remain inthis state, the non-detection of radio emission for six of the target sourcesallows us to constrain their black hole mass to the IMBH regime, thus rulingout a supermassive black hole nature. For the extreme ULX in the galaxyNGC\,2276, we detect extended radio emission formed by two lobes of total fluxdensity 1.43 $\pm$ 0.22 mJy and size $\sim$650 pc. The X-ray counterpart islocated between the two lobes, suggesting the presence of a black hole with jetradio emission. The radio luminosity allows us to constrain the black hole massof this source to the IMBH regime; hence, the extreme ULX in NGC\,2276 could bethe first detection of extended jet radio emission from an IMBH. The radioemission could also possibly come from a radio nebula powered by the ULX with aminimum total energy of $5.9 \times 10^{52}$ erg, thus constituting the mostpowerful and largest ULX radio nebula ever observed.

Radio observations of extreme ULXs: revealing the most powerful ULX radio nebula ever or the jet of an intermediate-mass black hole?