Plaid Cymru MP Jonathan Edwards has attacked the party's leader Leanne Wood for showing "no leadership" on Brexit.
Ms Wood is facing a challenge to her position from assembly colleagues Rhun ap Iorwerth and Adam Price.
Speaking to WalesOnline, the MP claimed Ms Wood was "more concerned about the niche issues that interest her" and did not have a sense of how important Brexit is.
Ms Wood said she was "disappointed" by recent public criticism.
A letter sent on Friday by the party leader hit out at politicians making statements in the media before going through "proper channels".
It follows Mr Edwards' support for a proposal by pro-European Conservative MP Anna Soubry that Plaid could help form a cross-party "government of national unity" to negotiate Brexit.
Ms Wood has repeatedly ruled out working with the Conservatives at all levels.
Ms Soubry's comments came after she joined 11 other Tories in rebelling against Theresa May on Brexit in the House of Commons on Tuesday night.
Responding at the time to the suggestion, Mr Edwards, MP for Carmarthen East and Dinefwr, said if stopping the UK from leaving the European Union's single market and customs union required his party to enter an all-party government "so be it".
WalesOnline reported that his comments "infuriated" Ms Wood.
Mr Edwards, who managed Ms Wood's leadership election campaign in 2012, was quoted on the website as saying: "I am afraid that Leanne has shown no leadership in relation to Brexit since the referendum result. As MPs, we have been left to our own devices in Westminster void of any direction by the leader.
"We are faced with an unprecedented situation where the best-case scenario from the UK government's point of view is a hard Brexit and the worst-case scenario would involve a no-deal crash-out with the worst Brexit conceivable that would be cataclysmic for the Welsh economy.
"It's vitally important that Plaid shows national leadership at this crucial time, but Leanne doesn't seem to have any sense of how important it is.
"We have to be prepared to seize the initiative and work with those who will help deliver our objectives. The idea put forward by Anna Soubry would involve all parties. Clearly the Tories would have to be involved for mathematical reasons. This is not the time for tribalism.
"Leanne seems more concerned about the niche issues that interest her, but what Wales needs in this desperate crisis is real leadership."
There has been no direct response to Mr Edwards' comments, which also followed criticism by Adam Price that Leanne Wood cannot be the next first minister.
But a letter seen by BBC Wales that was sent to all Plaid politicians by Ms Wood said she was "disappointed to read various political interventions this week", and that "proper internal channels have not been used" before statements were issued that were damaging to the party.
"The current leadership election should not be an excuse for some to have a scorched earth policy that will make it harder for the party to come back together after the election," she said.
A party source took exception to the fact that all party politicians were included in the letter.
"These are men who were once perceived as Leanne's closest allies and men she has relied upon.
"Other members are not treated this way. If someone says something in public that is contrary to what we deem acceptable, then it should be flagged with them," the source said.
"Perhaps she needs to reassess who her allies are or are not."