Labour at 'crucial point' in its history, Wayne David MP claims
The Labour Party is at a "crucial point" in its history and could "cease to exist" unless its leadership row is settled, according to Caerphilly MP Wayne David.
He said it was time for party leader Jeremy Corbyn to step down.
But Mr Corbyn has said MPs needed to "respect" the views of the party members who had elected him.
Meanwhile, Newport West MP Paul Flynn has been appointed Shadow Welsh Secretary.
He takes over from Nia Griffith who resigned from the position, and comes after several MPs quit the Labour front bench in the wake of the EU referendum vote for Brexit.
Mr David was among those to quit and told BBC Radio Wales that Mr Corbyn's position was "untenable" after a no confidence vote by his MPs.
Speaking to the Sunday Supplement programme, Mr David said he and colleagues stood down "because they are concerned that the Labour Party is not an effective, coherent opposition under Jeremy's leadership".
"We have done our level best to work with Jeremy," he said.
"Jeremy's position is completely untenable and I really think that what he should do is look at the likely scenarios over the next few months and years and recognise that, though a decent man he is, his position as leader of the Labour Party has effectively come to an end.
"This is a crucial point in the Labour Party's entire history.
"If the Labour Party does descend into total civil war then it is quite possible that the Labour Party may cease to exist."
Rhondda MP Chris Bryant is also concerned about the future of the party and urged Mr Corbyn to "go out with dignity".
He told the BBC's Sunday Politics Wales programme he could not imagine any other leader of the Labour Party, apart from Ramsay MacDonald, who would not have taken on board the result of the motion of no-confidence.
"I'm sure in Jeremy's heart, he knows that there is a real danger that his broken leadership will break the Labour Party," he said.
"If he is listening, please, please, please Jeremy, you're the only person who can break this log jam."
Mr Corbyn has refused to resign, saying the motion of no-confidence - a vote of 172 to 40 - has "no constitutional legitimacy".
But former Labour leader Lord Neil Kinnock said party rules mean Mr Corbyn would need to secure backing from more than 50 MPs if he wanted to fight a leadership challenge.
Lord Kinnock told BBC One's Andrew Marr Show: "Unless the leader can have that substantial support in Parliament then there should be a contest or the leader should consider his position and do his duty to the party and resign."
Later on Sunday, Mr Flynn confirmed he has been appointed as Shadow Welsh Secretary.
It comes as the Wales Bill is set to be discussed on Tuesday, and Mr Flynn said he believed UK and Welsh ministers could come to an agreement.
"It has a reasonable compromise and we should sit to discuss it as mutually intelligent people with a great deal of respect for both sides," he said.
"It will be a rare event and a sensible discussion, not with grandstanding."
He also said he agreed with Mr David and Mr Bryant's comments but said to stand down would leave Plaid and SNP able to "takeover".