Former Taoiseach John Bruton says he is in favour of retaining the Eighth Amendment.
He claims there will be no protection for the unborn in the Constitution if the Eighth is repealed and the Dáil will be able to legislate.
The public is expected to vote in a referendum on abortion this summer.
People will be asked if they are in favour of terminations without restriction up to 12 weeks into pregnancy - and afterwards if there is a risk to the mother.
John Bruton has said he is firmly against it.
He said: "We should adopt the precautionary principle if there is any possibility that we could be inflicting pain, I think we should avoid doing so.
"I believe that when an abortion is taking place, particularly if it's a late-term abortion which new Government legislation will allow in certain circumstances, I'm almost certain that the child will suffer pain in the process of being killed."
The former Taoiseach is warning that people will not stop buying abortion pills online, if we legalise terminations, and also says Ireland should be proud that we protect the right to life.
Pro-choice campaigners claim it could reduce the number of women ordering abortion pills over the internet, but Mr Bruton does not believe it will make a difference.
He said: "The fact that there are deficiencies in the enforcement of any law doesn't mean you shouldn't have the law.
"People ordering those pills over the internet, that's a problem in Britain as well and hasn't been cured by having fairly liberal abortion law, so I think that remains a problem."
It comes as the Irish Examiner reports that doctors will be unable to legally provide abortion pills for up to seven months after any legislation is passed — potentially blocking availability until summer 2019.
, reports that the Health Products Regulatory Authority (HPRA) confirmed that strict licensing rules mean doctors will not be allowed to dispense abortion pills except in rare circumstances for up to 210 days after a bill allowing them is signed into law.
Speaking as former president Mary McAleese said the Eighth Amendment is not working as planned and that evidence to remove it is “compelling”, the HPRA said if the referendum and 12 weeks laws are passed abortion drugs will not be readily available for several months.
The group told the Irish Examiner this is because doctors will be unable to provide the medications except in rare circumstances as they will have no licence.
“No medicine for the termination of pregnancy is currently authorised in Ireland,” said a spokesperson.
“The process for authorisation of a medicine begins with a company submitting an application dossier containing quality, safety and efficacy data.
"Should the review of this dossier result in a positive benefit-risk assessment, an authorisation for placing the medicine on the Irish market could be granted. There are limited circumstances under which such products can be legally prescribed. The HPRA has no remit on policy issues relating to constitutional matters."
A country cannot allow the sale of a drug until the producer successfully applies for a national licence or EU licence under the European centralised licensing system. However, in both cases, HPRA sources said the licensing assessment process for new and existing drugs can take up to seven months or 210 days.
While doctors can apply for access to an unavailable drug from a different country through an “exempt products” rule while the licensing assessment takes place, GP concerns over insurance and other matters mean it is unclear how regularly such a step will be used.
And, coupled with Taoiseach Leo Varadkar indicating on Sunday that any post-referendum 12-week law will not be debated until the autumn, it means abortion pills are unlikely to be available until at least summer 2019.
Meanwhile, Mrs McAleese said obstetricians such as Dr Peter Boylan have given “compelling” evidence to repeal the Eighth Amendment and that it may have been “too tightly drawn”.
Saying “this law is not operating well”, Mrs McAleese said obstetricians’ concerns need to be taken “seriously”.
“As somebody who did back in the day support the Eighth Amendment, I have to say I’m disappointed with how it rolled out. Sometimes that happens with laws, the words get in the way.”