Mark Drakeford says Boris Johnson should use the election successes of Labour in Wales and the SNP in Scotland to “reset relationships” across the United Kingdom.
The Welsh Labour leader said there were “very real tensions” within the union, and said he wanted assurances from the Prime Minister that they could work together to preserve Wales’ place in it.
Mr Drakeford is set to continue as Wales’ First Minister after Labour equalled its best ever Senedd election result by winning 30 seats – just one short of a majority.
He has previously called for an “entrenched form of devolution” where Westminster cannot roll back powers transferred to devolved administrations.
Speaking in Porthcawl, Mr Drakeford told the PA news agency: “This really is a moment that the Prime Minister should seize to reset relationships across the United Kingdom, for a serious examination of the way in which we can create the machinery that will allow us to work together in the future.
“Not an approach that thinks flying more Union Jacks at the tops of buildings, but proper, respectful relationships that recognise that sovereignty is now dispersed across four parliaments in which we choose to pool it for common purposes.
“That’s the sort of UK that I think will have the very best chance of surviving, because it will be a UK where people want to be here, rather than are instructed to be.”
Following Labour’s unlikely comfortable victory in the Welsh Parliament election, Mr Drakeford said his preference was to work with other parties to form a new government.
Labour can choose to form a minority government, but inviting members of other parties into a Labour-led administration would give the party greater chance of pushing through its agenda.
“We’ve demonstrated in the past that you can govern successfully with 30 seats, but my approach will be to work with other parties where there are policy ideas that we have in common,” Mr Drakeford told PA.
“No party has a monopoly of good ideas. I’m much more interested in working with others where we think that will be to the betterment of Wales than I am in the sort of political fixing of things.
“I’m looking forward to working with anybody who thinks that by doing things together we can do things better.”
Mr Drakeford said the contrast between Labour’s election performance in Wales and England was down to the record of Welsh Labour governments over their 22-years in power.
“The public know that of we say something in our manifesto, then they can have confidence that it will be delivered,” he said.
Asked why he thought the Welsh public had so just trust in his leadership, Mr Drakeford said: “What I hope people see is I am what I am. I’ve never believed in politics that you should pretend to be something that you’re not.”
He said he would celebrate his election success on night with his wife Clare as well as his daughter, son in law, and his grandson who were “bringing fish and chips with them.”
Earlier Mr Drakeford vowed to be “radical” and “ambitious” in government.
Asked if he planned to continue his cautious brand of politics during a new administration, Mr Drakeford told PA: “Well, absolutely as far as coronavirus is concerned. The pandemic has not gone away.
“A government I lead will continue to follow the science to do what our medical advisers tell us we should do, and that does mean doing things in a way that continues to keep Wales safe.
“But on other matters, our manifesto is a radical manifesto with a host of ideas that are ambitious for Wales.”
He said his government’s post-pandemic priorities would be rebuilding the NHS, investing in the futures of young people, and putting Wales economy on an “ambitious” path that uses the country’s strengths the basis of its future direction.
With the final results in, the Welsh Conservatives have 16 seats, while Plaid Cymru has 13 and the Liberal Democrats have one.
Welsh Lib Dem leader Jane Dodds, who won a regional seat in Mid and West Wales after her party lost the Brecon and Radnorshire constituency to the Conservatives, said she has yet to be approached by Mr Drakeford to help form the next government.
She told BBC Radio Cymru: “I need to speak to other people within the party and we shall have to see.”
The Welsh Conservatives said they had secured the party’s “best ever result” in a Senedd election, winning 16 seats.
This included taking the Vale of Clwyd from Labour, and Brecon and Radnorshire from the Liberal Democrats.
Andrew RT Davies, Senedd leader for the Welsh Conservatives, said he was “delighted” to have secured those constituency seats as well as an increased number of seats on the regional list.
“It’s been an unconventional campaign and it’s clear incumbency and continuity has played a significant part,” Mr Davies said.
He congratulated Mr Drakeford and Welsh Labour on a successful campaign and said the election had been fought “in good spirit” by political parties across Wales.
Plaid Cymru now has 13 seats in the Welsh Parliament, though high-profile former leader Leanne Wood lost her Rhondda seat to Labour.
On her Facebook page, Ms Wood said the result was “disappointing” but that her team could “hold our heads high in the knowledge that we ran a clean and honest campaign, we did not denigrate our opponents and we worked hard”.
Current Plaid leader Adam Price also congratulated Mr Drakeford, and said on Twitter that he was looking forward to “continue to build the case for independence”.
“Westminster’s attack on devolution is only just beginning and Wales needs a plan – that plan must focus on taking our own future into our own hands so we can build a nation that is fair and free,” he said.