In a statement, clinicians said: "The constitution of NicoBloc is such that it is classified as a food additive and is derived from corn syrup. There is therefore no requirement for NicoBloc to be clinically tested or licensed under the regulations of the Medicines Control Agency.
"The question regarding setting a date for women to stop smoking rather than encouraging them to continue is ill-informed. The whole purpose of using NicoBloc is to start an immediate process to enable an individual to gradually withdraw from their dependency on nicotine and importantly without the need for nicotine patches or nicotine substitutes, in other words NicoBloc is nicotine free.
"The trial taking place using NicoBloc is focused on women volunteering to take part. The Trust also provides a traditional smoking cessation support service for pregnant women who choose not to take part in the trial."
A spokeswoman for NicoBloc added: "Giving up smoking is one of the most important things that people can do for their health, and this is particularly true with regard to pregnant women.
"However, prevalence of smoking amongst expectant mothers is high. Pregnancy can be a very stressful time for some women and setting an abrupt smoking cessation date can intensify this stress for mother, baby and family.
"Only 3pc of people stop smoking successfully when they rely on willpower alone "NicoBloc, however, offers preg-nant women a safe and natural solution should they be unable to quit through willpower alone.
"Research also demonstrates that, contrary to other opinion, smokers using NicoBloc on their cigarette filters do not draw harder to compensate for the lost level of nicotine."