  We report on 1.6 and 5.0 GHz observations of the ultraluminous infrared
galaxy (ULIRG) Mrk 273, using the European VLBI Network (EVN) and the
Multi-Element Radio-Linked Interferometer Network (MERLIN). We also make use of
published 1.4 GHz VLBA observations of Mrk 273 by Carilli & Taylor (2000). Our
5 GHz images have a maximum resolution of 5-10 mas, which corresponds to linear
resolutions of 3.5-7 pc at the distance of Mrk 273, and are the most sensitive
high-resolution radio observations yet made of this ULIRG. Component N1, often
pinpointed as a possible AGN, displays a steep spectral index ($\alpha = 1.2
\pm 0.1; S_\nu \propto \nu^{-\alpha}$); hence it is very difficult to reconcile
with N1 being an AGN, and rather suggests that the compact nonthermal radio
emission is produced by an extremely high luminous individual radio supernova
(RSN), or a combination of unresolved emission from nested supernova remnants
(SNR), luminous RSNe, or both. Component N2 is partly resolved out into several
compact radio sources --none of which clearly dominates-- and a region of
extended emission about 30 pc in size. The integrated spectral index of this
region is flat ($\alpha = 0.15 \pm 0.1$), which can be interpreted as due to a
superposition of several unresolved components, e.g., RSNe or SNRs, whose radio
emission peaks at different frequencies and is partially free-free absorbed.
The overall extended radio emission from component N is typical of nonthermal,
optically thin radio emission ($\alpha = 0.8 \pm 0.1$), and its 1.4 GHz
luminosity ($L_{1.4 \rm GHz} = (2.2 \pm 0.1)\times 10^{23} $ WHz$^{-1}$) is
consistent with being produced by relativistic electrons diffused away from
supernova remnants in an outburst.
