The Irish Republican party confirmed their MPs were attending the House of Commons induction day for newcomers, register for allowances and sign up for office space.
Sinn Fein historically opposes Westminster’s jurisdiction in Northern Ireland and its oath to the Queen, so its MPs have abstained from sitting in parliament.
The Tories’ failure to secure an overall majority in the House of Commons has given the DUP’s 10 MPs a potentially crucial role in keeping Mrs May’s administration afloat.
Sinn Fein said in a statement: “They have elected us to represent them but not to take our seats.
“We will come over to Westminster to argue with other parties and to fight for our rights and to fight for the rights of Irish citizens.”
If Theresa May fails in her bid to get the Queen’s speech through Parliament it is possible that Mr Corbyn could have the chance to form a minority Labour Government himself.