document: The Portadown Times published results from what it said was an "independent" poll ahead of May's general election.
  But it failed to acknowledge that the survey had actually been commissioned by the Democratic Unionist Party (DUP).
  Mr Nesbitt said the article "appears to have skewed" the election result.
  The article suggested that the gap in support between the DUP and Sinn Féin had narrowed.
  The results put the DUP candidate David Simpson in first place with 30.7% of the vote, with Sinn Féin in second place on 28.1%.
  The Ulster Unionist candidate, Jo-Anne Dobson, was third in the poll, with 25% of the vote.
  Mr Simpson held his House of Commons seat for Upper Bann in the election with 15,430 votes, with Mrs Dobson polling 13,166 votes to finish in second place.
  The Ulster Unionist Party said the publication of the article suggested to readers that a vote for Mrs Dobson would have would "split the pro-union vote, and risk the seat being taken by Sinn Féin".
  The Independent Press Standards Office (IPSO) upheld Mr Nesbitt's complaint that the newspaper's description of the poll as "independent" had misled readers.
  IPSO said the Portadown Times had also failed to inform readers that the poll had been conducted six weeks before it was published.
  Mr Nesbitt said his party's canvassing team felt the publication of the "out-of-date" survey had "provoked a dramatic mood-swing on the doorstep".
  "I have no doubt David Simpson will do the decent thing and call a by-election, so the people of Upper Bann can vote without the artificial shadow of fear hanging over their ballot papers," he added.
  But DUP MLA Sydney Anderson said the Ulster Unionists needed to "accept the verdict of the people".
  "Every candidate is entitled to disappointment following an election defeat," Mr Anderson said.
  "What we are witnessing from the Ulster Unionist Party, however, is sour grapes on an industrial scale."
summary: Ulster Unionist leader Mike Nesbitt has called for a by-election in Upper Bann after a press standards body ruled voters had been misled about a poll published in a local newspaper.