The TV gardener has agreed to be the new face of B&Q and is also about to start filming a BBC series — Alan's Garden Secrets.
The BBC and B&Q said Titchmarsh is not endorsing B&Q products but simply offering advice to customers.
But Taxpayers Alliance chief executive Matthew Elliott said: "It's time the BBC decided whether it wants to be a public broadcaster with presenters not able to make money through commercial deals or be a commercial broadcaster with no licence fee."
Titchmarsh said: "I thought long and hard about signing up with B&Q. But they reach more potential gardeners than any other outlet in the country."
A BBC representative said: "We have guidelines to ensure we avoid commercial conflicts of interest. The new Titchmarsh series will be made in accordance with these guidelines, which, as we understand it, his agreement with B&Q does not breach."