The large current uptake of e-cigarettes (2.8 million U.K. users in 2016; 26), the continuous involvement of e-cigarettes (which potentially affects their contents as well), the uncertainty about their medium- and longer-term effects, and the need to promote smoking cessation as means of reducing Cardiovascular disease, dictate that more research is urgently needed. Research exploring the impact of e-cigarettes on cardiovascular function/ health has been requested by the European Parliament, the British Medical Association, regulatory agencies, clinicians and researchers, as there is currently no consensus within the smoking cessation community as to the potential impact of e-cigarettes. With e-cigarettes being successful in replacing traditional cigarettes (i.e. up to 42% within a month), such studies should not only be efficacy-focused, but should also explore the physiological effects of e-cigarettes, as preliminary work in the field is contrasting and limited, in both the acute- and longer-term timeframe. Furthermore, as e-cigarettes are not simple nicotine-based products, the general public, researchers and government agencies cannot rely on the existing research exploring the effects of nicotine on the cardio-vasculature (e.g. coronary and peripheral vasoconstriction, intravascular inflammation and deregulation of cardiac autonomic function as well as inhibition of microcirculation). Thus, the lack of direct evidence - which would clarify the degree of safety of e-cigarettes for the user's vascular system and determine their longer-term cardiovascular disease risk - increases the need for research in the field.

Such studies will supply in-depth information to service-users and policy-makers, especially as the recently-initiated U.K.'s "Medicines and Healthcare Products Regulatory Agency" validation of e-cigarettes will increase likelihood of their introduction in smoking cessation clinics.

This study will bridge the existing knowledge gap and inform the general public, the scientific and the smoking cessation community in respect to the physiological (e.g. cardiovascular health) and participants' experience of both nicotine-inclusive and nicotine-free, e-cigarettes (when combined with behavioural change support) and compare it against a currently NHS-applied smoking cessation pathway that combines Nicotine Replacement Therapy and behavioural change support. This will allow future research to advance and optimize the pharmacological treatment of tobacco and nicotine dependence, by taking into consideration the study's physiological and Health Economics' findings.

Inclusion Criteria:

• smokers (at least 10 cigarettes/ day for the past year)
• willing to attempt quit smoking by using the National Health Service smoking cessation services or via e-cigarette use.

Exclusion Criteria:

• Non-ambulant people and people with a recent (e.g. within 6 months) cardiovascular disease event (e.g. stroke, myocardial infarction) or cardiac surgery,
• people with insulin-controlled diabetes mellitus or with coexisting skin conditions, leg ulcers, vasculitis or deep venous occlusion (as these may affect their cardiovascular function),
• pregnant women,
• people who require major surgery (which will prevent them of taking part in the study)
• people who following referral to the smoking cessation services, are advised that Nicotine Replacement Therapy is not appropriate for them,
• people who are currently using e-cigarettes on a daily basis, or currently undertaking an attempt within a smoking cessation service,
• people who are judged to be unable or are unwilling to give informed consent.

The study team will assess prospective participants and will not include in the study those who appear too frail or have health problems (e.g. dementia, Alzheimer's etc.) that might prevent them of giving a conscious, informed consent.