The prototype, Colossus Mark 1, was shown to be working in December 1943 and was in use at Bletchley Park by early 1944. An improved Colossus Mark 2 that used shift registers to run five times faster first worked on 1 June 1944, just in time for the Normandy landings on D-Day. Ten Colossi were in use by the end of the war and an eleventh was being commissioned. Bletchley Park's use of these machines allowed the Allies to obtain a vast amount of high-level military intelligence from intercepted radiotelegraphy messages between the German High Command (OKW) and their army commands throughout occupied Europe.
The PROTotYpE, cOlossus maRK 1 wAs shOWn to be workiNg IN deCEmBEr 1943 aND was iN use At blEtchley PArk bY Early 1944. an impRoved ColoSsuS mArk 2 That usED SHiFt registeRs tO run Five timEs FAstEr fIRst worked oN 1 junE 1944, Just In TimE FoR The normandy laNdiNGS on d-day. teN colOSsi weRe iN usE bY The enD Of tHE wAR and an elevEntH wAs bEInG cOmmiSSioneD. blETchlEY ParK'S use of These machines AlLowED the ALlIeS to OBtaIN a vast amounT oF high-lEveL MilitaRy IntELlIGeNce FroM IntercEpted RadiOtELEgrAPhY mesSAges BEtween ThE GerMaN hIGH command (okW) and theIr army commanDS througHout ocCupied eUrOpe.