The outfit, called SMLXL, will work with clients that are having problems reaching consumers because of the changing media landscape.
Chaldecott will be joined in the venture by Alan Moore. He is a brand strategist who has worked on projects for Coca-Cola, helped relaunch the Renault Clio in the UK, evolved a global brand communications strategy for Saab and devised the US store opening strategy for the H&M fashion chain.
For Chaldecott, the venture marks his reappearance 15 months after his decision not to follow HHCL into its merger with WPP's Red Cell network. At the time he said: "I have a few hot irons in the fire that I haven't been able to fully explore while being part of HHCL."
The launch of SMLXL takes place against a background of what its founders say is the lack of effectiveness of tried and trusted means of communication as new media proliferate and the number of TV channels increases.
The pair point out that not only are mass television audiences declining, but viewers are finding commercials more intrusive and less satisfying.
Meanwhile, TV advertising clutter is increasing and revenues are down, they claim.
"TV will continue to grow in importance but we must find new ways to exploit the medium," Chaldecott said.
The company's solution is what it calls "engagement marketing", which enables brands to appeal to audiences both emotionally and intellectually.
Chaldecott and Moore cite the huge public enthusiasm for Pop Idol and the Guinness Visitor Centre in Dublin as examples of this current trend.
Chaldecott, 49, built his creative reputation at the then GGT and WCRS before becoming a founding partner of HHCL in 1987.
Despite having a lower profile than HHCL's other creative founder, Steve Henry, his output helped define the agency's iconoclastic work, which polarised the ad community throughout the late 80s and early 90s.
As well as being involved in HHCL's most famous campaign, in which a bald, fat orange man slapped the faces of Tango drinkers, Chaldecott helped launch First Direct, the low-cost airline Go and spearheaded the relaunch of the AA as "the fourth emergency service".