The Norwich MP Clive Lewis has warned of "Conservative domination" if more of his colleagues quit the Labour Party.
Seven MPs resigned yesterday (Monday 18 February) over disillusionment with Jeremy Corbyn’s politics.
Mr Lewis, the MP for Norwich South, said: "It weakens the Labour Party and it makes more likely more years of Conservative domination and Conservative Government.
"I don’t think anyone who considers themselves a progressive would think that is a good thing."
Mr Lewis added: "The gang of four left in the 1980s and we had the SDP and arguably that helped usher in a decade more of Margaret Thatcher and the Conservatives.
"Now from my political perspective that was a disaster for this country and I wouldn’t want to see this Conservative administration, or the MPs who currently sit here on the Conservative side, to have another 10 years in Government given the disaster they’ve managed to make of the Brexit process and getting us into this mess in the first place."
Mr Lewis also said by-elections should be held in the seven constituencies where MPs had quit the Labour Party.
He said: "I think there’s a legitimacy to that because many of the people in that constituency, in those seven constituencies, voted not necessarily for those individual candidates, but for the Labour brand they represented, and that means that brand is no longer represented in that constituency."
Mr Lewis said the former Labour MPs should not set up a new party.
He said: "I think it will be the wrong thing to do because ultimately they will split votes between progressives.
"They will take votes away from the Labour Party and they will make it easier and more likely that Conservative candidates, if they decided to stand in marginals for example, had more chance of winning, because the so called progressive vote is split amongst the electorate, and that doesn’t bode well for people in this country in my opinion."
Mr Lewis urged other MPs not to walk away from the Labour Party.
He said: "My appeal to them would be: let’s work together, lets talk, let’s keep those communications open.
"Don’t walk away because the only people that will benefit will be our political opponents and the people that will suffer will be the people that we came into politics to try to change their lives for the better."