An MP suspended by Labour for remarks about the party's handling of anti-Semitism has again lost the whip - two days after being readmitted.
Chris Williamson was suspended in February after saying Labour had "given too much ground" over the issue.
However, he was allowed back in on Wednesday after an investigation by the party's National Executive Committee.
Now, a Labour source says issues raised by an MP on the investigation panel needed to be examined.
"Subsequently, the whip is not restored as the decision is still pending," the source said.
Mr Williamson tweeted: "I'm naturally concerned by the lack of due process and consistency in how my case is being handled", adding that he would do his "utmost" to fight for his membership.
Labour's general secretary Jennie Formby has written to the NEC to inform it of the issues with the investigation raised by the panel member - Labour MP Keith Vaz - and his concerns will be on the agenda for its next disputes committee meeting, said the source.
"Under the party's rule book, the general secretary and the leader of the party cannot overturn decisions made by NEC panels, which are advised by independent barristers," the source said.
"Only the NEC has the power to do so."
An action replay?
Analysis by Iain Watson, BBC political correspondent
It was Keith Vaz - a former minister who is not seen as politically close to Chris Williamson - who had the pivotal vote over whether to readmit the Derby North MP.
He is now calling the process into question and suggesting the whole thing could be rerun - an action replay that could give a different result and defuse a potentially explosive internal row.
Mr Vaz was brought into the process at the last minute and the theory being advanced by some in leadership circles is that he initially and mistakenly did what he thought Mr Corbyn wanted, the better to avoid a deselection threat.
When he saw the strength of the backlash, he suggested effectively rerunning the process.
But Mr Corbyn is in a difficult position - one his internal opponents relish.
There was a large backlash from Labour peers and MPs after Mr Williamson was allowed back into the party on Wednesday.
Deputy leader Tom Watson was among more than 100 of them to sign a letter calling for Labour Party leader Jeremy Corbyn to act and remove the whip from the Derby North MP.
Seventy-one Labour politicians also wrote to the chair of the Parliamentary Labour Party (PLP), John Cryer, earlier, asking him to begin a process that could have lead to the whip being withdrawn from Mr Williamson if Mr Corbyn did not act.
After his readmission Mr Williamson had said he "deeply" regretted the remarks and did not want anyone to think he was "minimising the cancer of anti-Semitism".
He told BBC Radio Derby: "Anybody who knows me, who knows my record, knows I'm someone who has stood up against bigotry throughout my political life and indeed beforehand."