Top Labour figure Emily Thornberry has accused MPs who quit her party to form the new Independent Group of leaving so that they can “cuddle up to the Tories”.
The shadow foreign secretary told a party rally in Nottinghamshire that she “would rather die” than join any other party.
At the same rally leader Jeremy Corbyn also warned that the departing MPs had made a “bad mistake” and suggested they did not understand the origins of the Labour Party.
Nine MPs quit the party in recent days, with eight joining three ex-Conservative members to create a new group.
Ms Thornberry told the rally: “We have a great deal in common and the biggest thing we have in common is the fact that we are Labour.
“We are Labour to the core and Labour to the tips of our fingerprints – and we would rather die than join any other party and we would never think of joining the other eight people who have decided to abandon Labour and cuddle up to the Tories.”
Those who have left so far include Chuka Umunna, Luciana Berger, Mike Gapes, Gavin Shuker, Ann Coffey, Chris Leslie, Angela Smith, Joan Ryan and Ian Austin, though Mr Austin has not joined the new grouping.
The Labour rally on Saturday was in Beeston, in the Broxtowe seat of Anna Soubry – one of the ex-Conservative MPs in the new group.
Mr Corbyn said to the crowd: “I’m obviously 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. Not understanding where we have come from is a bad mistake.
“Because when people come together in a grouping, in a community like the Labour Party, there’s nothing we can’t achieve together for everybody.”
With Westminster rife with rumours of further resignations, Mr Corbyn did recently hold out an olive branch to his critics with a hint the party is edging closer to backing a second EU referendum.
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