A senior Labour MP has called on Theresa May to step aside and "let us have a go" at forming a government.
Llanelli MP Nia Griffith - the shadow defence secretary - said Labour had to be ready "at any moment" to take over.
The Democratic Unionists are still in discussions over a possible deal with the Tories, who lost their majority in Thursday's election.
Wales Office Minister Guto Bebb said "the only option" was a Tory government with support from other parties.
Downing Street said the details of the outline deal would be discussed at a cabinet meeting on Monday.
Labour has suggested it could form a minority administration, but with 262 seats falls far short of the 326 needed for a majority in the House of Commons.
Leader Jeremy Corbyn has ruled out striking a deal with other parties.
Ms Griffith told BBC Radio Wales: "What we really want to be about is making sure we can try, if we can, to put some of our ideas into practice, and that means being a very united group in parliament.
"It means being ready, at any moment, to take over and step into government whenever there might be a chance.
"We're still saying quite clearly that we would be ready to take over if Theresa May cannot cobble something together, and we're very doubtful that she can.
"We don't really think she has a mandate to now, she's really made a fool of herself and really, quite frankly, she should step aside and let us have a go."
Mr Corbyn had given his party "confidence" about selling party policies such as public ownership of the railways, she said.
On Thursday, Labour won back three seats from the Conservatives in Wales - Cardiff North, Gower and Vale of Clwyd.
Ms Griffith said voters had switched to Labour during the election campaign itself.
"People have a real sense of community in Wales," she said.
"They like Labour ideas like looking after public services and I think people came together, they saw what Theresa May was offering, which was very empty - I can't really call it a manifesto - and they saw her arrogance and almost disdain for people, and they really did not like that.
"What was interesting was the number of people who made up their minds during the campaign; we saw things swinging at the very last minute."
Last June, Ms Griffith joined a Labour rebellion against Mr Corbyn following the referendum vote for Brexit, resigning as shadow Welsh secretary in protest at what party critics claimed was his half-hearted campaign in favour of remaining in the European Union.
She returned to the front bench in October as shadow defence secretary after Mr Corbyn defeated a challenge to his leadership from Pontypridd MP Owen Smith.
Mr Bebb dismissed the idea that Labour could take power.
"It's very clear to me that the only option on the table, to be perfectly frank, is a Conservative government supported by some other parties in Westminster," he said.
"The DUP have voted on a fairly regular basis with the Conservative Party in the past so the nature of any agreement is something for the prime minister to work out.
"But it's not surprising that there is a possibility of a Conservative minority government being formed with the support of the DUP. I don't find that in any way surprising."