Private-equity group CVC Capital Partners - the lead member of the consortium - brought in M as details of its interest in making Europe's biggest-ever private equity deal leaked to the media earlier than intended.
M said CVC has charged it with emphasising that the consortium's interest may not result in an actual bid.But if a mooted £9bn offer for the UK's number-two supermarket does materialise, M will handle the interest that will result.
The account represents the first time M agency founders Nick Miles and Hugh Morrison have worked together for a client since they launched their consultancy in 2002.
The agency, which has not worked with CVC in the past, has dedicated five staff to the brief in total.
With the potential deal being so large - and Sainsbury's perceived as something of a national institution - M will also have to counter negative associations with private-equity deals.
Sainsbury's is using its retained City adviser Finsbury as it prepares a defence.