Ed Sheeran has denounced anti-abortion campaigners after reports they used one of his songs less than a week before Ireland’s referendum on abortion.

The British pop star’s song Small Bump was reportedly being played by anti-abortion activists campaigning in Dublin’s city centre. Sheeran distanced himself from those using his song, saying in a post on Instagram that “it does not reflect what the song is about”.

Sheeran’s Instagram post. Photograph: Ed Sheeran

Small Bump, released in 2011, is from Sheeran’s debut album +, and his objection comes as Ireland prepares to vote on Friday 25 May in a referendum to repeal the eighth amendment to its constitution, which bans abortion in almost all circumstances.

The song’s lyrics include: “You’re just a small bump unknown, you’ll grow into your skin/With a smile like hers and a dimple beneath your chin/Finger nails the size of a half grain of rice, and eyelids closed to be soon opened wide/ A small bump, in four months you’ll open your eyes.”

In his Instagram post, Sheeran, who is currently playing a series of concerts in Ireland, said: “I’ve been informed that my song Small Bump is being used to promote the pro-life campaign, and I feel it’s important to let you know I have not given approval for this use, and it does not reflect what the song is about.”

Ireland’s referendum has attracted interest from groups from around the world seeking to influence the vote. A significant proportion of Facebook posts aimed at influencing voters were shown to have come from pages managed partly or entirely outside Ireland. Even some groups registered with Ireland’s ethics watchdog, the Standards In Public Office Commission, had Facebook pages managed from abroad.

Facebook said it was blocking all foreign spending on advertising around Ireland’s referendum on abortion in an effort to adhere to the “principles” of the country’s election spending laws.