An attempt to legalise same-sex marriage in Northern Ireland and loosen its strict abortion laws is to be launched by MPs this week, the Observer has learned.

The parliamentary ambush, which could be backed by senior Conservative MPs, is set to cause a major headache for the government. Ministers have insisted that the issues should be left to Northern Ireland’s power-sharing assembly, but with the assembly not currently sitting MPs have been plotting a way to force through a change in the law.

Attempts to loosen abortion laws in Northern Ireland have been accelerated since the landslide result in Ireland’s referendum in favour of ending its near total ban on abortion. It has left the region isolated, with the UN raising concerns about laws that campaigners say put women’s safety and dignity at risk. The region has some of the most restrictive laws in the world, allowing abortion only where the woman’s life is at risk or to save her from permanent psychological or physical damage. Meanwhile, same-sex marriage has also been allowed in the Irish Republic, but remains illegal in Northern Ireland.

Labour MPs Stella Creasy and Conor McGinn have drawn up amendments to an obscure Northern Ireland bill currently being rushed through parliament. Votes are currently expected to take place on Wednesday.

Their “equal rights” amendment will create a problem for many Tories who back the changes. Tory MPs Anna Soubry, Huw Merriman and Nicky Morgan are among those known to want a change in the law in Northern Ireland on abortion. Forcing through any change creates political problems for Theresa May, who continues to rely on the support of Northern Irish Democratic Unionist (DUP) MPs who oppose a change in the law.

Recent polling has shown that 76% of people in Northern Ireland support equal marriage, with just 18% opposed. Meanwhile, 65% of Northern Irish people think abortion should be decriminalised and 66% think Westminster should act in the absence of the assembly.

Creasy said: “The inequality in basic human rights between the people of Northern Ireland and their fellow citizens in the rest of the UK is a travesty. The prime minister has repeatedly promised MPs a free vote on these issues – it’s now time to see if she puts principle or staying in power with the DUP first.”

McGinn said: “Every poll shows that equal marriage has the support of the vast majority of people in Northern Ireland, most of the political parties there and the overwhelming number of MPs in the House of Commons. Equality for LGBT people in Northern Ireland is long overdue, and this amendment gives the government the opportunity to match its words with deeds and bring Northern Ireland into line with the rest of the UK.”