A blog about a Labour candidate who is now suing its author for libel was published with a "clear view to the public interest", a court has heard.
Writer and publisher Stephen Walker said the conduct of Anna Turley, who was Labour MP for Redcar, had given "reasonable grounds for suspicion".
He told the High Court in London that the 2017 story on his Skwawkbox site had been "particularly newsworthy".
Ms Turley, 40, is suing Mr Walker and the Unite union.
She says the article, which contained a press statement from Unite, libelled her by conveying the meaning that she had acted dishonestly in submitting an application to join the union.
In a written witness statement, Mr Walker said a source had told him a "flood" of new members had joined Unite Community on "the concessionary rate" who were ineligible because they were employed.
He was given Ms Turley's name and decided to publish, he said. He had not been "procured" by Unite, he added.
"What made it particularly newsworthy, in my view, was the allegation of an MP, amongst others, joining illegitimately on the concessionary rate rather than the correct rate for someone in employment," he said in the statement.
"Her conduct in joining at the concessionary Community rate... gave reasonable grounds for suspicion."
There had been a "lack of serious harm" to Ms Turley's reputation, he told the judge.
Unite bosses and Mr Walker are fighting the case and say Ms Turley has been dishonest and is not fit to be an MP.
They say she wanted to vote against Unite general secretary Len McCluskey and undermine Labour leader Jeremy Corbyn without the union knowing she was an MP.
The court heard Mr Walker was a Labour Party member and a supporter of Mr McCluskey and Mr Corbyn.
The trial continues.