  Delayed "pair echo" signal from interactions of very-high-energy gamma rays
in the intergalactic medium can be used for detection of the inter-galactic
magnetic field (IGMF). We use the data of Fermi/LAT telescope coupled with
LHAASO observatory measurements to confirm the presence of IGMF along the line
of sight to the gamma-ray burst GRB221009A. Comparing the Fermi/LAT
measurements with the expected level of the pair echo flux, set by the
multi-TeV LHAASO detection, we derive a lower bound $10^{-19}$ G on the IGMF
with correlation length $l$ larger than 1 Mpc, improving as $l^{-1/2}$ for
shorter correlation lengths. This provides an independent verification of
existence of a lower bound on IGMF in the voids of the Large Scale Structure,
previously derived from the observations of active galactic nuclei.
