Chief marketing and communications officer Weed called into question the impact of the "Turn to 30" activity, which is based on laundry brands such as Ariel.
Weed said: "Just telling people to wash at a lower temperature sounds good, but getting people to do it is another challenge. People either forget or are economical with the truth."
His comments follow market research carried out by Unilever, in which consumers placed electronic chips in their washing machines to measure the temperature and length of washes.
Unilever has now unveiled plans to halve the carbon footprint of its products and source all agricultural raw materials sustainably within 10 years.
The FMCG has set out more than 50 economic, social and environmental targets to reduce the amount of greenhouse gas emissions and water and waste generated by itself, its suppliers and consumers of its products.