Neither Gavin Anderson nor the government of Luxembourg would comment on the appointment.
Arcelor retains Publicis and sister financial comms firm Capital MS&L but has also brought in Finsbury to handle its defence against Mittal Steel's £12.7bn bid.
Mittal brought in The Maitland Consultancy to handle PR around its offer (PRWeek, 3 February).
Arcelor is one of three companies which each account for around 20 per cent of the exchange's main index, the Luxx. The other two are European financial services giant Fortis and satellite company SES Global.
Although each of the trio's part of the Luxx is restricted to 20 per cent, a delisting would have a significant effect on the exchange and could undermine its competitiveness in a consolidating European exchange market.
Luxembourg's exchange is under pressure from its neighbouring and larger rivals Deutsche Boerse in Germany and pan-European exchange Euronext. Arcelor is also listed in Paris, Brussels and Madrid.
Luxembourg ministers have expressed concern at the bid's hostile nature after Arcelor's board rejected Mittal's offer two weeks ago.