Jess Phillips is set to pull out of the race to be the next Labour leader.
The Birmingham Yardley MP is expected to release a statement later amid speculation she will announce she is withdrawing from the contest.
She missed a hustings earlier on Tuesday organised by the GMB union, at which the other four candidates pitched for the union’s endorsement.
The contest is in its second phase, with candidates seeking the backing of unions and local Labour parties.
So far, shadow Brexit Secretary Sir Keir Starmer is the only candidate to have qualified for the final ballot – which begins on 21 February.
To make it to the final stage, the candidates have to secure nominations from 5% of constituency Labour parties (CLPs), or three affiliate groups – two of which must be trade unions – representing at least 5% of affiliated members.
Sir Keir became the first to clear this hurdle after being backed by Unison, the UK’s largest union, and a second union, Usdaw.
At the moment, Ms Phillips does not have any unions or constituency parties in her corner.
The GMB will reveal who it is supporting later, having heard from Sir Keir, Lisa Nandy, Rebecca Long-Bailey and Emily Thornberry at a two-hour hustings in central London.
Speaking at the first regional hustings of the contest last weekend in Liverpool, Ms Phillips said Boris Johnson would be “terrified” by the prospect of facing her in the Commons.
She has insisted she has the “big personality” to change how Labour is perceived by the public after its fourth election defeat in a row, but she has struggled to gain momentum in the early stages of the race to succeed Jeremy Corbyn.
Mr Corbyn’s successor will be announced on 4 April.