Senior Labour figures fear the possibility of electoral wipeout at the hands of the Lib Dems in London, after the party lost its deposit in the Richmond Park by-election.

With Labour committed to delivering on Brexit, in part to appease the threat of Ukip in its pro-Brexit northern heartlands, a London Labour source told The Independent that several of the party’s MPs are now “terrified of the Lib Dems”, who have said they will contest any general election and by-election on a pro-EU basis.

“Ukip is not a massive issue in London but the Lib Dems are,” the source said. “We’re are between a rock and a hard place. That is where we are. We cannot see a way out of the bind.”

Labour received just 1,515 votes in the by-election, less than the 1,600 members the local party said it had. Aberavon MP and Corbyn critic Stephen Kinnock said: “The Witney by-election was disappointing because it emerged that we had doubled our membership but halved our share of the vote, and now we find in Richmond that there are more Labour Party members than there are Labour voters.

“This represents a worrying trend, and underlines how vital it is that we do more to reach out beyond our core vote.

“Recently published UK-wide opinion polls also indicate that Labour has not yet succeeded in building the broad support base that is a pre-condition for winning elections.

“In these difficult and uncertain times our country desperately needs a strong and effective Labour Party, holding the government to account and preparing to form the next government.”

Another troubling implication of the Liberal Democrats’s success is it showed the party campaigning machinery is functioning well.

Labour’s membership figures have sky-rocketed under Jeremy Corbyn but longer standing local party sources say this will not necessarily translate into effective electoral campaigning.

Several London constituency Labour parties have more than doubled in size since Corbyn’s election to the leadership, but it is not reflected in activism.

“It’s reflected in the number of people that will tweet about Jeremy Corbyn, or a vote on Trident,” one source told The Independent, “but it’s not reflected in people knocking on doors in marginal seats in London on a Saturday.”