The number of women travelling from Northern Ireland to have an abortion in England has jumped dramatically since the government set up a special hotline in March.

A total of 342 women and girls – including at least one 12-year-old – went to England for a termination through the British Pregnancy Advisory Service (BPAS) in the three months since March: a significant increase on the 190 women who travelled to use the same service in the previous nine months.

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The figures have been released as pressure mounts on the government to repeal 19th-century legislation that prevents women from having an abortion in Northern Ireland. It is now the only part of Europe apart from Malta where abortion is illegal, after the Irish referendum in May.

On Wednesday, Theresa May will meet Leo Varadkar, the taoiseach, for the first intergovernmental conference between Britain and Ireland since 2007.

A letter to the leaders, signed by 173 parliamentarians from every major political party in Ireland and the UK, calls for the repeal of sections 58 and 59 of the Offences Against the Person Act 1861. In England, Wales and Scotland, the legislation has been superseded by the Abortion Act 1967 and other legislation, but it does not apply in Ulster.

Campaigners including Stella Creasy, the Walthamstow MP, who is one of the letter’s signatories, say that having different abortion laws in Ireland and Northern Ireland is a breach of the Good Friday agreement, which commits both governments to having equivalent legislation on both sides of the border.

Creasy called the letter an unprecedented intervention. She said: “We cannot let the human rights of women of Northern Ireland be forgotten. Our duty under the Good Friday agreement is to protect them, not let Theresa May sacrifice them to the political expediency of having the DUP prop up her government.

“Without action it’s clear hundreds of women and girls as young as 12 who are UK citizens are being forced to travel overseas for healthcare – and many more may be forced to continue an unwanted pregnancy as a result. The government must name the date when parliament can repeal this cruel legislation or else risk making a rape victim having to take them to court to vindicate the basic human rights of Northern Irish women.”

On 6 March the Department of Health and Social Care set up a central booking system so that women in Northern Ireland could call a phone number to make an appointment through BPAS. The figures show that an average of 28 women a day crossed the Irish Sea for an abortion through BPAS between March and May, with more than half drawing income support or having an income of less than £15,276 a year.

In 2017, 919 Northern Irish women travelled to England for an abortion, most using clinics run by BPAS or Marie Stopes.

In June, the supreme court said the abortion law in Northern Ireland was not compatible with the European convention on human rights.