Consultant Bob Ivison said it was crucial for the sector to "collectively make representations" to the utilities and add their voices to those already lobbying hard to squeeze out exemptions such as watering of newly laid turf.
Ivison, who belongs to the National Contractors Forum, said: "This is not merely about back gardens. The forum represents around 50 companies such as Quadron and ISS, with a combined turnover of £670m and 12,600 staff."
His model letter, which he has sent to all member companies, states: "Without the use of hosepipes many of the companies will have no option but to cease operations and or lay people off."
This would have bad knock-on effects on the national economy while "this kind of a repeated financial hit will soon bring our companies to financial ruin", his letter states.
Ivison, said: "We understand the need for sensible use of water but it’s up to individual contractors to take a view on what needs to be watered and what exemptions they should go for."
Follow-up action would depend on the response from the water companies but the forum was keen to work closely with the Professional Gardeners’ Guild, he said.