Labour front bench MP Debbie Abrahams has stepped aside from her shadow cabinet post while she is investigated over an "employment issue", Labour has said.
But the shadow work and pensions secretary hit back, saying she had not agreed to stand down and criticising her treatment at the hands of the Labour leader's office.
The party gave no details of the allegations against Ms Abrahams, although it is understood she is the subject of a number of complaints, at least one of which concerns bullying.
She is to be the subject of a "no prejudice investigation" by the party, rather than the whips office, it is understood.
But in a statement, Ms Abrahams said: "I refute the allegations that have been made against me in the strongest possible terms. I will fight this spurious claim and do not rule out taking legal action.
"I have had no details about the complaint, who it is from, the process or timescales. I have not agreed to stand aside.
"My treatment by certain individuals in the Leader's Office over the last 10 months has been aggressive, intimidating and wholly unprofessional.
"My treatment in the last week has shown a bullying culture of the worst kind. As such I am making a formal complaint to both the Labour Party and Parliamentary Authorities.”
- Analysis by ITV News political correspondent Dan Hewitt:
A statement from Labour announced that Ms Abrahams would be temporarily replaced by her deputy Margaret Greenwood, but gave no further details.
Ms Abrahams first entered Parliament in 2010, winning the Oldham East and Saddleworth by-election after the election of Labour MP Phil Woolas was ruled void over false statements he made about a rival candidate.
She was made shadow minister for the disabled by Jeremy Corbyn in 2015 and was promoted to the shadow cabinet after last year's general election.