One in five politicians at Stormont was not elected into their post, it has emerged.

Of the 108 MLAs charged with running the country, 21 have been appointed by their parties to replace colleagues who have stepped down either through illness or to take up other political posts.

Substitute MLAs have now been appointed in 12 of the 18 Assembly constituencies, with four of the current six south Belfast Assembly members unelected.

The figure is set to rise further after Peter Robinson announced he is to stand down as First Minister, DUP party leader and MLA in a matter of months.

Legislation was introduced in 2010 which allowed parties to replace outgoing MLAs with a replacement of their choice.

The substitution system was put in place to cut by-election costs of up to £215,000 per poll.

However, the practice has come under scrutiny, most recently with the appointment of Emma Pengelly, a former special advisor to Mr Robinson, who received severance pay of £46,000 when she resigned from her £92,000 per annum post prior to joining the Assembly.

She replaced Jimmy Spratt as a DUP MLA for South Belfast at the end of September after he retired.

She was subsequently promoted to the post of junior minister, a position that attracts an annual salary of £60,000, just one month later. The leader of the Green Party in Northern Ireland, Steven Agnew, said there are occasions when the practice of co-opting is the most sensible and efficient route to follow.

He continued: "I do think where ill health is concerned, then there is a place for the system.

"When you look at examples such as Claire Sugden replacing David McClarty, that seems like the most sensible thing to do.

"However, we have a situation where MLAs resign, not on health grounds, who are replaced by party colleagues and it provides them with some time to raise their own profile before going in to an election and I do think that is abusing the system somewhat."

The research on the number of MLAs co-opted on to the Assembly was compiled by Detail Data, a collaboration between the detail investigative journalism website and NICVA.

It states multiple reasons for the large turnover of MLAs, with the eradication of double jobbing among political representatives one of the main drivers.

Fourteen MLAs have left Stormont during the current Assembly term to take up seats in Westminster, Brussels or local councils.

According to the law, a by-election is held if the party nominating officer fails to provide details of a substitute within a specified time, or if an independent candidate fails to submit a list of substitutes. This has not happened since the legislation came into effect.

An MLA can change party and retain their seat - as was seen when Basil McCrea and John McCallister left the UUP to set up N121. However, whilst an MLA is free to change party, the seat returns to the party if the politician subsequently vacates the post.

So, who replaced who on the Hill?

SDLP

Sean Rogers in for Margaret Ritchie

Claire Hanna replaced Alasdair

McDonnell

Fearghal McKinney in for Conall McDevitt

Sinn Fein

Christopher Hazzard in for Willie Clarke

Maeve McLaughlin replaced Martina Anderson

Megan Fearon in for Conor Murphy

Rosaleen McCorley in for Paul Maskey

Declan McAleer in for Pat Doherty

Bronwyn McGahan replaced Michelle Gildernew

Ian Milne in for Francie Molloy

Alex Maskey in for Sue Ramsey

Mairtin O Muilleoir in for Alex Maskey

Conor Murphy in for Mickey Brady

Independent

Claire Sugden in for David McClarty

DUP

Maurice Devenney in for William Hay

Gary Middleton replaced Maurice Devenney

Gordon Lyons in for Sammy Wilson

Emma Pengelly replaced Jimmy Spratt

UUP

Adrian Cochrane-Watson in for Danny Kinahan

Neil Somerville replaced Tom Elliott

Andrew Allen in for Michael Copeland

Belfast Telegraph