Published Friday, 14 December 2012
Declan Gormley outside the High Court in Belfast (© Presseye)
Declan Gormley was defamed in two separate press releases issued by the party, a jury held.
Following their decision a judge also rejected a further defence to the claim based on responsible journalism.
Mr Gormley will return to court next week to discover what level of damages he is to be awarded.
Legal costs of the three-week trial are also expected to run into hundreds of thousands of pounds.
The businessman, who was dismissed from the government-owned company by former Sinn Féin Regional Development Minister Conor Murphy, spoke of his relief at the outcome.
He said: "I'm delighted, I feel the verdict has vindicated the position I have held throughout that this was something that was wrong."
I'm also disappointed that I had to end up in court, I would have been happy to settle for an apology and a public acknowledgement that it was wrong.
Mr Gormley was sacked along with three other non-executive directors from NI Water in March 2010.
Mr Murphy removed them from the board following an independent review team investigation into the awarding of contracts.
A subsequent Stormont Public Accounts Committee report into procurement and performance at the government-owned company was said to have criticised the earlier inquiry and questioned its independence. Mr Gormley emphatically denies any wrongdoing.
He issued libel proceedings against Sinn Fein and two of its representatives, MLA Cathal Boylan and former Assemblyman Willie Clarke, over the contents of press releases.
Over the course of a 10-day hearing at the High Court in Belfast, his legal team claimed they damaged his reputation and were reckless if not dishonest.
Lawyers for Sinn Féin contended, however, that neither statement contained any defamatory content.
They are also claimed a defence of qualified privilege, arguing that the press releases were in response to a campaign of attacks on Mr Murphy mounted by the SDLP with Mr Gormley's collusion.
But a jury of five men and one woman found for Mr Gormley on the balance of probabilities.
They decided that both statements were defamatory and that the defendants were guilty of malice.
Mr Justice Gillen then rejected a final defence of qualified journalism, ruling that no steps had been taken to try to verify the contents of the press releases before publishing them.
He said: "I'm satisfied that the press releases in question fell below the acceptable standards of journalistic approach."
The judge held that no steps were taken to try to verify the defamatory claims made in the press releases.
He will now direct the jury on Monday to enter a formal finding on behalf of Mr Gormley.
They will also be asked to decide on what level of damages should be paid out.