It comes after the former Change UK MP joined Sir Vince Cable’s unashamedly pro-EU party, claiming last night that Labour and the Tories were “committed to facilitating Brexit”.
He acknowledged his role within the party could change after the conclusion of the Liberal Democrats’ leadership contest.
But frontrunner Jo Swinson said she had a spring in her step, claiming Mr Umunna’s decision to join the party demonstrated the Liberal Democrats are the “rallying point” for those wishing to stop Brexit.
Her leadership rival, Ed Davey, also welcomed Mr Umunna to the party, suggesting both contenders are unlikely to shift him from the role after the result of the party’s internal election next month.
Mr Umunna told The Independent: “Vince has asked me to lead on treasury and business issues in the House of Commons, pending the election of our new leader who will no doubt appoint their own team.
“I will dive straight in and get on with that in addition to, of course, continuing to serve my constituents in Streatham.”
Before defecting from Labour to Change UK, Mr Umunna had previously served as Ed Miliband’s shadow business secretary between 2011 and 2015.
Following disappointing European election results for Change UK – set up earlier this year and made up of disaffected Labour and Tory MPs – Mr Umunna went independent, alongside Heidi Allen and Sarah Wollaston.
The Streatham MP said the Liberal Democrats were now “at the forefront” of a “progressive and internationalist movement” in British politics.
“Labour and the Tories are committed to facilitating Brexit, and Brexit makes ending austerity virtually impossible,” he added. “The Liberal Democrats are not – they were arguing for a People’s Vote and to remain in the EU from the very start.”
Sir Vince, who will step down as Liberal Democrat leader next month, added: “Chuka and I have worked together effectively for many months, campaigning for a People’s Vote and to stop Brexit.
“I know that he will be a great asset to our party not just on Brexit, but in fighting for the liberal and social democratic values that we share.”
Labour immediately reissued its plea for a by-election in Streatham following Mr Umunna’s decision to join the Lib Dems.
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