News of his departure coincided with the announcement that a deal had finally been struck between WPP's group chief executive, Sir Martin Sorrell, and Lord Bell's Chime Communications for the sale of HHCL for a reported initial payment of about £4 million.
With his fellow founder, Steve Henry, poised to take creative control of the merged operation, Chaldecott said it was "time I move on and start something new".
The enlarged agency - to be called HHCL Red Cell - will claim billings of £165 million, including subsidiary operations, and bring together clients such as Unilever, Britvic, Iceland, Alfa Romeo, Singapore Airlines, Clinomyn and the Welsh Tourist Board.
Chaldecott will remain at the agency for the next few months to work on specific creative projects. But he added: "I have a few hot irons in the fire that I haven't been able to fully explore while still being part of HHCL."
The merged agency, comprising more than 100 people, will operate out of HHCL's London office, which is being seen as the "nerve centre" of the Red Cell network.
The signing of the agreement climaxed ten months of protracted negotiations thought to have been delayed in their final stages by talks to finalise Henry's contract.
Henry, regarded as HHCL's creative sparkplug, was seen as critical to the deal. But this week he said: "I am absolutely committed. I think we'll be the world's first global hotshop."
Red Cell has been desperate to make HHCL its passport to international credibility. Lee Daley, the network's chief executive, said: "Our need has been for a fantastic UK creative agency and that's what we're getting."
Under the deal, WPP will acquire 49 per cent of HHCL, taking full control once certain performance targets have been met. The cash will help cut Chime's £30 million debt.
Daley will also take on the chairmanship of HHCL Red Cell although Nick Howarth, HHCL's managing director, will be in day-to-day charge.
The deal will not involve either the HHCL chairman, Simon Burridge, who will remain at Chime as part of the company's central team, or the managing director of Red Cell's UK "shell" operation, Nick Kerr, who is leaving.
HeadlightVision, Red Cell's new product development, marketing consultancy and trend forecasting specialist, which is working increasingly on global business, will also move in to the HHCL office.
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