Caroline Lucas has hailed the Richmond Park by-election as a “step forward” for a so-called progressive alliance and credited her party with contributing to the Liberal Democrats victory.

The Green Party co-leader said the decision by the local Greens not to stand made a difference to the numbers and tone of the by-election, triggered by Zac Goldsmith’s decision to quit the Conservatives over the Government’s airport expansion plans.

At the general election in 2015 the Greens received 3,548 votes – or 6 per cent – in the constituency. Mr Goldsmith, who stood as an independent, lost the by-election by 1,872 votes.

Speaking to The Independent Ms Lucas said the Lib Dem victory in Richmond Park shows that when progressive parties are “willing and able to work together then we can win against the so-called regressive alliance”.

“It often feels as if the right of the spectrum are more organised at getting themselves together to maximize their chances under what is an incredibly dysfunction electoral system,” she added.

Asked about the degree of impact her party had in the by-election by not fielding a candidate, the Green leader replied: “I think it clearly made a difference – it made a difference in numbers and in tone because what we were doing was going out and talking to people about the need for a different kind of politics. A less tribal politics… if we can work together we can maximize our chances of success.

“The decision of the local Green party not to stand in the election was part of things that made a difference – it made a contribution.”

It is clear, however, that in order for the “progressive alliance” to work – and for a win in Richmond Park to be replicated across the country – then it needs Labour’s backing. But despite senior Labour MPs, including the shadow Business Secretary Clive Lewis, also calling for their party to give the Lib Dems a clear run in Richmond Park, the leadership decided to stick to protocol and provide the electorate with a candidate. They chose Christian Wolmar, a writer, broadcaster, and railway historian, who received 1,515 votes (3.6 per cent) and lost the party’s £500 deposit.

But Ms Lucas is hopeful that Labour’s leadership and the party HQ will change its attitude towards the “progressive alliance”. She added: “No one said it’s going to be easy – of course there is resistance. I think it’s significant you had three Labour MPs… coming together suggesting Labour should not stand a candidate in the Richmond by-election.

“Change doesn’t happen overnight but I think change is happening. It is interesting that John McDonnell as the shadow Chancellor has changed his position on electoral reform.

“More and more Labour people are realising they are not going to win an outright majority at the next election – and frankly, the result Labour got in Richmond will only reinforce that. Labour didn’t even keep its deposit.”

Ms Lucas, who alongside her fellow co-leader Jonathan Bartley, labelled Mr Goldsmith’s campaign as a “regressive alliance” after Ukip decided not to field a candidate – to the delight of the Lib Dems – in the affluent south-west London constituency.

“Recognising Zac as a principled man, who was fully committed to helping get Britain out of the European Union, Ukip leader Nigel Farage, in conjunction with our national executive committee, have agreed that we will not be fielding a candidate in the upcoming by-election for Richmond Park,” a statement from the party said at the time.

Ms Lucas continued: “This regressive alliance has been defeated and the Government has suffered a hammer blow to its hard Brexit plans.