Former attorney general Michael McDowell believes it would be unlawful for the Government to seek to impose blanket bans on legal firms who act for big tobacco tendering for legal services advertised by the State.
Last week, Minister for Children James Reilly said he is seeking legal advice on whether he can block certain legal firms from taking on work with his department.
The legal firms in question represent tobacco companies, which have threatened to sue the State if the Government continues with its legislative plan to introduce plain packaging on cigarettes sold in Ireland.
Mr McDowell said some recent media commentary suggested the Government effectively wanted to have nothing to do with legal firms acting for big tobacco.
Speaking on RTÉ's This Week, the former tánaiste said: "I don't think that would be lawful. I don't think that would be a reasonable condition for most tenders in relation to most legal services."
Mr McDowell, who is a barrister as well as a former minister for justice, cited the example of a firm of solicitors working for the State on a case around removing a child from its parents into State care.
He said this would have no connection with which side the same law firm may have acted for in relation to plain packaging for cigarettes.
He added that if there was an area of legal services that related to the direct consumption of tobacco, then there were potential conflict of interest questions for solicitors if they were also seeking State work around the implementation of preventative health programmes.
There has been considerable controversy around the subject since it emerged that law firm Arthur Cox, which works for the Health Service Executive, was also representing Japan Imperial Tobacco in its threatened legal action against the State in relation to plain packaging of cigarettes.
The same firm also acts for the recently established child and family support agency, Tusla, in Mr Reilly's department.
Mr McDowell said the Government could not say to firms of solicitors that if they have anything to do with a particular form of litigation, they would be "blacked" from certain forms of work if there was not a rational and defensible legal basis for doing so.
He said he was sure the current Attorney General would advise ministers on how far can they can go down the road of potentially excluding legal firms who act for big tobacco from accessing Government work.
Mr McDowell said that personally he had a good deal of sympathy with Mr Reilly's position on plain packaging.
He said he thought the perception that the Government was being threatened not to proceed with its legislative programme on plain packaging of tobacco had annoyed some ministers.