Google has banned abortion adverts in the run up the Irish referendum over fears overseas campaigners are trying to sway the vote.

Facebook has also limited advertising sales to people inside Ireland after a transparency tool accidentally revealed large numbers of anti-abortion posts were being posted from abroad.

Ireland will go to the polls on May 25 to vote on whether to appeal the Eighth Amendment, which makes abortion illegal unless the mother's life is in serious danger.

The Yes vote is currently leading in the polls but they show 20 per cent of voters are still undecided, which leaves the final result wide open.

Google has banned abortion adverts in the run up the Irish referendum over fears overseas campaigners are trying to sway the vote. Facebook has also limited ad sales to people within Ireland. Pictured: Pro-choice campaigners march in Dublin

The tech companies' ban on abortion ads comes after an accidental slip up last Friday.

For a brief period during the day a new tool designed to boost transparency over political ads in light of the Cambridge Analytica scandal went live on Facebook.

It enabled users to see the real-time location of people managing campaign pages, showing abortion ads coming out of the UK, US and Hungary among other foreign countries.

The social network quickly turned off the tool and said it had been launched by mistake.

Ireland bans political donations from abroad, but the law does not apply to social media advertising.

When the controversy came to light both tech giants decided to impose restrictions.

An anti-abortion group based in Leamington Spa paid for an ad about an Irish woman who died after travelling to the UK for an abortion.

Aisha Chithira, 32, died six years ago when being discharged from a Marie Stopes clinic in Ealing following a termination of a 22-week pregnancy.

Ireland will go to the polls on May 25 to vote on whether to appeal the Eighth Amendment, which makes abortion illegal unless the mother's life is in serious danger. Pictured: Supporters for the Yes campaign in Ireland

'This is not what care for women looks like,' the advert said. Facebook said it had violated the new rule when contacted for comment and removed the ad.

The controversial adverts were collected in a database created using software which trails through Facebook for information on why advertisers are targeting the user.

It comes after it was revealed anti-abortion groups have targeted young Irish voters on Facebook with claims that abortion makes women suicidal and causes breast cancer.

Parents also raised concerns that YouTube - which is owned by Google - has allowed videos aimed at children to be published with adverts that suggest one in five pregnancies are terminated in England.

Google's ban has been attacked by anti-abortion groups, who insist they face bias from traditional media and need social media to combat it.

David Quinn of Catholic thinktank the Iona institute recently tweeted: 'Of course mainstream media dislike social media advertising by pro-lifers. Such ads are a way of doing an end-run around media bias.

'Google are worried that the Yes side would blame them if they lose [the referendum].

'Obviously they couldn't care less if it has the opposite effect. That alone shows terrible bias.'