The agency won the account following a pitch handled directly by EnCams. The charity is understood to be planning to hire a further three agencies to work on advertising campaigns.
HTW will handle a range of marketing communications, including leaflets, brochures and poster activity. In recent years, EnCams has used its posters to drive consumers online to learn more about its initiatives.
Earlier this year, the charity, which is partly funded by the government through the Department for Environment, Food and Rural Affairs, ran a Keep Britain Tidy campaign during the FIFA World Cup. The nationwide activity comprised a series of posters with slogans such as '40 Years of dirt' and 'Eng-urgh-land! Eng-urgh-land!'
The charity also works alongside the Home Office on issues such as anti-social behaviour.
EnCams administers other environmental campaigns, such as the Blue Flag scheme in England and Northern Ireland, which gives awards for clean beaches, and an Eco-School drive that runs in 23 countries across Europe. Its other initiatives include campaigns to combat noise pollution, plus chewing gum and cigarette litter.
In August, the charity launched a cinema and poster campaign to encourage consumers to dispose of their rubbish properly. The cinema execution, created by Lake Design, showed rats scurrying over a bed.