Crayon is expected to retain its name and to continue as a division in its own right with its own clients.
The agency launched in 2005 and its clients include Avis, BT and UKTV.
Key clients at HS&P, previously Hicklin Slade & Partners, include Norwich Union Direct, Honda, Diageo and Camelot.
There are not expected to be any redundancies as a result of the merger, which will take HS&P up to 70 staff in total.
The deal, which hasn't been signed yet, is intended to strengthen HS&P's group offering which includes data specialist data.HS&P, dedicated consultancy Loyalty Practice and word-of-mouth agency Legends, which launched in Janaury.
Each business can work independently or alongside the main agency.
HS&P is headed up by founder and chief executive Justin Hicklin and managing director Matthew Brown.
Keith Hunt, managing partner at Results International Group, said: "We are going to see an increase in mergers of this sort. During a downturn people are looking at ways of reducing costs whether these be voluntary or forced on them, as clients are lost.
"Combining under one roof and under one brand name is one of the most significant things an agency can do to increase profitability.
"Mergers can also be an excellent means of an agency broadening its skill sets. In difficult times an increasing number of clients are rationalising their agency rosters down to a smaller number. For this reason agencies increasingly need to be able to deliver a range of services and an enhanced offer. This applies to digital in particular."
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