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We present the results of a series of Swift and quasi simultaneousground-based infra-red observations of the blazar 3C 454.3 carried out inApril-May 2005 when the source was 10 to 30 times brighter than previouslyobserved. We found 3C 454.3 to be very bright and variable at all frequenciescovered by our instrumentation. The broad-band Spectral Energy Distribution(SED) shows the usual two-bump shape (in nu-nu f(nu) space) with the Infra-red,optical and UV data sampling the declining part of the synchrotron emissionthat, even during this extremely large outburst, had its maximum in thefar-infrared. The X-ray spectral data from the XRT and BAT instruments are flatand due to inverse Compton emission. The remarkable SED observed implies thatat the time of the Swift pointings 3C 454.3 was one of the brightest objects inthe extragalactic sky with a gamma-ray emission similar or brighter than thatof 3C 279 when observed in a high state by EGRET. Time variability in theoptical-UV flux is very different from that in the X-ray data: while the firstcomponent varied by about a factor two within a single exposure, but remainedapproximately constant between different observations, the inverse Comptoncomponent did not vary on short time-scales but changed by more than a factorof 3 between observations separated by a few days. This different dynamicalbehaviour illustrates the need to collect simultaneous multi-frequency dataover a wide range of time-scales to fully constrain physical parameters inblazars.Comment: 7 pages, 4 figures, accepted for publication in A&

Swift and infra-red observations of the blazar 3C 454.3 during the giant X-ray flare of May 2005