Chuka Umunna has been appointed Treasury and business spokesman for the Liberal Democrats just days after the former Labour and Change UK MP joined the party.

Umunna announced his decision to join the Liberal Democrats on Thursday, 10 days after leaving Change UK and four months after leaving the Labour party.

The leader of the Liberal Democrats, Sir Vince Cable, announced this weekend that he was appointing Umunna as the party’s Treasury and business spokesman.

“Chuka has agreed to work in the Commons with our business and economy team, alongside Susan Kramer and Chris Fox who speak on these issues in the House of Lords,” said Cable. “His experience as shadow business secretary will be invaluable to the party in pursuing our shared values.”

Senior Lib Dem MPs including Jo Swinson, the frontrunner to be the next Lib Dem leader, and Ed Davey, the other contender in the leadership battle, were quick to welcome Umunna to the party last week. In the hours following his move the Liberal Democrats’ head of membership and engagement reported a surge of 800 new members.

In Umunna’s south London constituency this weekend, many of his supporters remain undeterred by his latest political move, including the antiques shop owner Steve Wright, 67, who said: “I think they should just leave him alone. He’s doing a good job for Streatham anyway, so why rock the boat and elect somebody else?”

Wright said he did not think Umunna should face a byelection in Streatham as he felt a general election was imminent anyway. “I would probably vote for him because he’s a man of principle. He’s doing a good thing to follow his heart. I think people should vote for candidates based on who they are and what they do, rather than which political party they follow the whip for.”

The designer Alex Lamb, 33, said he understood Umunna’s reasons for quitting Labour. “What he did was very brave. In my lifetime, it’s pretty unprecedented. At least they [the Change UK defectors] were trying to find the centre ground in a quite fractured time. I think it’s quite admirable that he admitted he made a mistake.”

Play Video 1:00 Chuka Umunna on his decision to join the Lib Dems – video

Umunna was elected as a Labour MP in Streatham in 2010, but quit the party to co-found Change UK, in part as a reaction to Labour’s antisemitism crisis and stance on Brexit. He abandoned the new group after they received only 3% of the vote in the EU elections.

However, others were incredulous that their MP had been allowed to change political parties twice without resigning. Erica Martin, 42, said: “I think we, as the constituents, should have a say on whether he remains our leader. At the moment the Conservatives are choosing a new leader to be prime minister and it’s the same thing – we don’t have a say as voters.

“There’s no clear standard of policy for anyone to follow. Where does he stand? It’s like he doesn’t know his own mind.”