A group of MPs who left Labour have "betrayed" their seats and would be "crushed" if by-elections were held, the shadow foreign secretary has said.
Emily Thornberry accused eight MPs who quit the party to form The Independent Group in the Commons of going to "cuddle up to Tories on the benches".
She told a Labour rally in Broxtowe she would rather die than join a new party.
Labour's deputy leader Tom Watson has said the defections are a cause for regret and reflection, not anger.
The departing Labour MPs said earlier this week they were leaving the party over its stance on Brexit and the leadership's handling of anti-Semitism.
They were followed by three Tory MPs who cited "a shift to the right" in their party and the government's "disastrous handling of Brexit" as reasons for their departure.
The newly-formed "The Independent Group" - which at the moment remains a grouping in Parliament not an official political party - says it represents "the centre ground of British politics".
They have urged like-minded MPs from other parties to join them.
But Ms Thornberry accused the Labour MPs of having had "the cheek to reject our new manifesto and our new leader".
She said: "It was our manifesto and our leader that gave them the huge majorities that they now have in their seats - those seats they have betrayed by their actions."
"If our new independent splitters have got the guts to have by-elections, we will crush them."
If an MP changes or leaves the party they were elected under, it does not automatically trigger a by-election. This is because at the ballot box voters choose the individual they want as their MP, not the party they wanted running the country.
A by-election can happen if the defectors resign as MPs or if voters in their constituencies call for a petition to recall their MP.
However, voters can only call for a petition under specific circumstances, such as an MP being convicted of an offence and receiving a custodial sentence. None of these conditions applies to the members of the Independent Group.
Labour leader Jeremy Corbyn also addressed the crowd in Broxtowe, Nottinghamshire - the constituency of the former Conservative MP Anna Soubry, who has also joined the new grouping.
"I'm very sad at some of the things that have happened and very sad at some of the things that have been said.
"Walking away from our movement achieves nothing", he said.
On Friday, Ian Austin become the ninth MP to quit Labour this week, blaming Mr Corbyn for "creating a culture of extremism and intolerance".
But the MP for Dudley North said he had no plans to join The Independent Group.
Meanwhile, Theresa May's government's working majority was reduced after three MPs resigned earlier this week - Ms Soubry, Heidi Allen and Sarah Wollaston.
The trio - who all support the People's Vote campaign for another EU referendum - held a press conference and criticised the government for letting the "hard-line anti-EU awkward squad" take over the party.
They joined The Independent Group, with Ms Allen saying she was "excited" about the future and wants to be "a part of something better, a party that people vote for because they want to, not because they feel they have to."
The Independent Group is not a registered political party and has not published a manifesto - although it does have a list of values on its website.
Now with 11 members, the new group has the same number of MPs as the Lib Dems and is the joint fourth largest.