The five Sinn Fein MPs elected in 2010 have never taken their seats in the House of Commons.

This has made it easier for the government to pass legislation, for instead of needing a minimum of 326 MPs to vote through its bills (326 being the notional threshold for a majority in a house of 650), it has needed only 323 (the number needed in a house of just 645).

Sinn Fein’s decision to boycott this parliament has been in keeping with its long-standing tradition of not sitting at Westminster. The party does not recognise the legitimacy of the UK parliament – though this attitude does not extend to a literal boycott of the premises, whose facilities Sinn Fein does use.

The very first woman elected to the British parliament was a Sinn Fein MP: Constance Markievicz, who won the seat of Dublin St Patrick’s at the 1918 general election. But she refused to take her seat in a gesture of support for her party’s abstentionist policy, setting a precedent to which all her successors have adhered.

And now it seems that precedent is set to continue for another parliament. On Friday, the party’s veteran campaigner and Northern Ireland deputy first minister Martin McGuinness confirmed Sinn Fein would boycott Westminster after the 2015 general election.

McGuinness conceded that some in his party had questioned whether this was sensible given the likelihood of another hung parliament and the opportunity to even hold the balance of power.

But he stated: “We have made it absolutely clear that our position in relation to not taking our seats won’t change.

“I know there will be an argument amongst some people in the event that things are very, very close that those votes could be influential in terms of the formation of the next government… [but] I don’t envisage any circumstances in which Sinn Fein would be taking their seats.”

This means that if Sinn Fein holds the five seats it won in 2010, the magic number for an absolute majority in the new House of Commons will once again be 323, not 326.

In turn, this will make the business of forming the next government slightly easier for David Cameron or Ed Miliband. In the event of needing to piece together a new coalition, even the tiny difference between a target of 323 rather than 326 will count for a lot. This is particularly true if both the Tories and Labour fail to make it above 300 seats, and the Liberal Democrats are reduced in number by up to a third: all of which have been suggested by recent opinion poll trends.

It could also make the prospect of a minority government slightly more palatable for the main party leaders, if only as a short-term arrangement ahead of a second general election.

So despite not being in parliament in person, Sinn Fein could still end up making its presence felt.