PRWeek understands that Mars recently contacted six agencies inviting them to send in credentials. Mars is still putting together a brief and expects to draw up a shortlist by mid-June.
The confectioner, whose brands include Snickers, Galaxy and Twix, hopes to have an agency in place by August.
Grayling has held the account spanning corporate affairs, public affairs and trade PR since 1984. UK chairman Vivien Hepworth said: 'We can't complain after a relationship of this length if they want to have a look at what's in the marketplace. But we are energetically and enthusiastically repitching for the business.'
Hepworth declined to say how many Grayling staff worked on the account.
It is understood that the review has been triggered by the recent arrival of Alison Clark as director of corporate affairs. She joined Mars from News International, where she was director of business development.
Industry insiders said Mars was a 'serious client' and estimated the account would be worth around £250,000.
One of the big challenges for the successful agency will be responding to the growing concerns about obesity, especially among children. Last year Mars announced a self-imposed global ban on advertising snack foods and confectionery to children under 12.
Mars has just completed an £11.5bn takeover of chewing gum group Wrigley, making it the largest worldwide confectioner. The merged Mars-Wrigley dominates global confectionery with a 14.4 per cent market share, ahead of Cadbury with 10.1 per cent and Nestlé with 7.7 per cent, according to Euromonitor.
Mars' other PR agencies, JCPR and Ketchum, will not be affected by the review.