The prime minister is set to warn of a "backroom stitch up" between the Conservatives, Reform UK and Plaid Cymru ahead of next year's Senedd elections.
Sir Keir Starmer will address Welsh Labour's annual conference in Llandudno, North Wales, on Saturday.
Voters will head to the polls next May to choose their representatives in Cardiff Bay and recent polls suggest Labour is in third place, behind Reform and Plaid.
Labour has been the largest party at every Senedd election since devolution began in 1999.
It is understood he will say such a deal would mark a "return to the chaos and division of the last decade".
But opposition parties have hit back at the prime minister's "imaginary coalitions", with Plaid Cymru accusing Labour of "scraping the barrel".
Sir Keir is expected to tell members how the two Labour governments in Westminster and Cardiff are "working together for the people of Wales".
At next year's Senedd election, 96 members will be elected to the Welsh parliament for the first time - an increase of more than a third - under a more proportional voting system.
He is anticipated to describe First Minister Eluned Morgan as a "fierce champion" for Wales and "the best person" to lead the country into the future.
It comes after Ms Morgan set out the "red Welsh way" in a landmark speech last month, seemingly deviating from the Westminster party, and calling for more "respect" for devolution and a "fair deal" for Wales.
The prime minister's pitch to the people of Wales comes as the UK government announces legal protections for military personnel, their families and for veterans as the country marks Armed Forces Day.
Further details are set to be outlined in due course, but the government says they could include more travel benefits or flexible working arrangements for partners of serving personnel.
After a week in which the government U-turned on controversial welfare reforms amid a mounting rebellion, the party is expected to make a number of announcements for Wales.
Among those is a new £11m fund for businesses in Port Talbot and the surrounding area, which Labour says proves that it is the only party with "a real plan to grow Welsh industry".
(c) Sky News 2025: Prime Minister Sir Keir Starmer set to warn of 'backroom stitch up' between parties in Wales
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