Ms Aziz was an outspoken member of the Al-Jazeera team, opposing controversial new staff contracts and pushing for union representation for the group. She was given notice on 1 January.
The English-language web site has a team of 11 journalists, many recruited from the BBC and the Guardian. Senior editor Yvonne Ridley, who helped to launch the news site earlier this year, was sacked without warning in November.
"Shaista Aziz brought in a rich seam of exclusive stories that were followed up by the international media," said Ms Ridley.
"She was outspoken, feisty and extremely professional."
Jihad Ballout, spokesperson for Al-Jazeera, told dotJournalism that the decision was made purely on the basis of Ms Aziz’s professional ability.
"I have absolute respect for my ex-colleagues," he said.
"My understanding is that the editor-in-chief made an assessment after the three-month probation and decided that perhaps she was not quite the right person."
However, it has been claimed that several members of the newsroom staff were present when Ahmed Sheikh, her immediate boss, told her recently that she had successfully passed her probation.
Recently Ms Aziz had made a formal complaint of 'Islamophobia' and sexual harassment against a male colleague. This case will not now be dealt with.
"There is a culture of fear and intimidation running through Al-Jazeera's English web site," said Ms Ridley.
"Those journalists who stand up to this also happen to be the most talented. I'm afraid journalistic integrity and skill appear to count for nothing."
Staff at the Doha-based broadcaster were said to be devastated by the sacking of Ms Ridley, an experienced and well-respected journalist of more than 25 years.
Ms Ridley has given power of attorney to her lawyers to take Al-Jazeera to court for unfair dismissal.
"Despite my unfortunate experience, I still have lots of respect for the Al-Jazeera name and those talented journalists on the web site and television who strive for journalistic excellence," she said.
"But there are people employed in the newsrooms and in the administration department who have no real journalistic background.
"They are to journalism what Herod was to child care."
The sacking is likely to fuel speculation that Al-Jazeera had been pressurised by the United States to tone down its news content. Staff on the English-language site have alleged that news items were withdrawn after phone calls from the White House.
"Everything in the world media is under pressure from the US government," said Mr Ballout.
"But Al-Jazeera has never succumbed to pressure."