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We present an observational study of the luminous red nova (LRN) AT\,2021biyin the nearby galaxy NGC\,4631. The field of the object was routinely imagedduring the pre-eruptive stage by synoptic surveys, but the transient wasdetected only at a few epochs from $\sim 231$\,days before maximum brightness.The LRN outburst was monitored with unprecedented cadence both photometricallyand spectroscopically. AT\,2021biy shows a short-duration blue peak, with abolometric luminosity of $\sim 1.6 \times 10^{41}$\,erg\,s$^{-1}$, followed bythe longest plateau among LRNe to date, with a duration of 210\,days. Alate-time hump in the light curve was also observed, possibly produced by ashell-shell collision. AT\,2021biy exhibits the typical spectral evolution ofLRNe. Early-time spectra are characterised by a blue continuum and prominent Hemission lines. Then, the continuum becomes redder, resembling that of a K-typestar with a forest of metal absorption lines during the plateau phase. Finally,late-time spectra show a very red continuum ($T_{\mathrm{BB}} \approx 2050$ K)with molecular features (e.g., TiO) resembling those of M-type stars.Spectropolarimetric analysis indicates that AT\,2021biy has local dustproperties similar to those of V838\,Mon in the Milky Way Galaxy. Inspection ofarchival {\it Hubble Space Telescope} data taken on 2003 August 3 reveals a$\sim 20$\,\msun\ progenitor candidate with log\,$(L/{\rm L}_{\odot}) =5.0$\,dex and $T_{\rm{eff}} = 5900$\,K at solar metallicity. The aboveluminosity and colour match those of a luminous yellow supergiant. Most likely,this source is a close binary, with a 17--24\,\msun\ primary component.

Observations of the luminous red nova AT 2021biy in the nearby galaxy NGC 4631