A prisoner with health problems has taken his fight to make smoking in jails a crime to the UK's highest court.
Paul Black, an inmate at HMP Wymott in Leyland, Lancashire, wants the ban on lighting up in public places to apply to state prisons.
The Supreme Court hearing follows his defeat in the Court of Appeal.
The government says a "vigorous" ban could affect safety and security, and is "phasing in" smoke-free prisons.
Five justices headed by president Lady Hale are considering whether the Crown is bound by the 2006 Health Act, which includes the ban on smoking in public places introduced in 2007.
Black's lawyers have said the sex offender, who has been serving an indeterminate sentence since 2009, suffers from problems including hypertension and heart disease due to second-hand smoke.
Black won a High Court case in 2015 that the legal ban must also be applied to state prisons and other Crown premises in England and Wales.
However, a Court of Appeal challenge by the government last year was allowed after lawyers warned a "particularly vigorous" ban could cause discipline problems and risk the safety of staff and inmates.
When given permission to take his case to the Supreme Court, Black said: "I simply wish non-smoking prisoners and prison staff to have the same level of protection as non-smokers living in the wider community."
Philip Havers QC, for Black, told the justices: "It is submitted that it is clear from the terms of the legislation itself that Parliament intended the Crown to be bound by the smoking ban."
He argued that it was not only prisoners being deprived of the protection of the Act, saying visitors would include children "who are particularly at risk from passive smoking".
The Prison Service is rolling out smoke-free prisons gradually "in a safe and secure way".
A spokeswoman said: "This will reduce the risk to staff and prisoners of exposure to second-hand smoke."
The hearing in London is expected to conclude on Wednesday.