The Liberal Democrat victory in the Richmond Park by-election is a "wake up call" for UK ministers and politicians, the party's Welsh leader has said.
Liberal Democrat Sarah Olney ousted ex-Conservative MP Zac Goldsmith after campaigning on the issue of Brexit.
Welsh Lib Dem leader Mark Williams said voters were saying they did not want a so called "hard Brexit".
The Conservatives have said the result "doesn't change anything" and ministers remain committed to leaving the EU.
Mr Goldsmith resigned his seat and stood as an independent in the southwest London constituency after the government backed a third Heathrow runway.
Celebrating overturning a 23,015 Tory majority, Mr Williams said Welsh Liberal Democrats were "proud" to have worked hard on the campaign.
"This win is not just about Richmond Park, but it is an important statement from the electorate that the government cannot continue to take us down the road to a hard Brexit, which would see us far poorer outside of the single market," he said.
"That would be bad for people in Richmond Park, and it would be disastrous for people in Wales, whether in towns and cities, or in small rural communities."
He added: "Sarah's victory must be a wake up call for both the government and politicians of all parties."
The Conservative Party said in a statement: "This result doesn't change anything.
"The government remains committed to leaving the European Union and triggering Article 50 by the end of March next year.
"In addition, we will continue to take decisive action in the national interest to secure the UK's place in the world."