The new first minister turned down a one-to-one meeting with Theresa May to get to a Labour celebration in Cardiff.
Mark Drakeford was to meet the PM at 14:00 BST on Wednesday, but her office said it had to be “pushed back”.
He was offered an early evening slot but BBC Wales understands he declined due to a “prior engagement in Cardiff”.
That event was a party to thank people who worked on his Welsh Labour leadership campaign. The Welsh Government has been asked to comment.
Mr Drakeford and Mrs May did meet at the Joint Ministerial Committee, which also included the Scottish first minister and Northern Irish officials, where the implications of a no-deal Brexit were amongst the issues discussed.
But the one-to-one would have been the first between the two leaders since Mr Drakeford was appointed first minister of Wales last week.
A source who was at the party has told BBC Wales that Mr Drakeford told them, “Jeremy Corbyn knows about you as you ran a fantastic campaign to get me elected.
“Theresa May didn’t know about you but she does now as I told her I had to come home to meet you all. She offered me a one-to-one but I said I had a prior engagement with you all.”
Plaid Cymru MP Jonathan Edwards called it “a shocking abdication of responsibility.”
“On the day that marked a hundred days to Brexit, the first minister thought it more appropriate to quaff champagne with his mates rather than make Wales’ voice heard in Westminster.”
“To put your political party before your country is shameful, to put a celebratory party before your country is madness.”
Leader of the Conservatives in the assembly Paul Davies said it was “not a good start” for Mr Drakeford.
“It is crucial that we have a committed individual who will go that extra mile to ensure that Welsh people are represented at the highest level.
“I appreciate that he had other commitments but surely he could have rearranged his party as discussions with the prime minister should take precedence.”
A Number 10 spokesman said Mrs May and the first minister spoke earlier this week on the phone but were unable to hold a bilateral meeting on Wednesday because of diary clashes.
He said a meeting was now planned for “early in the new year”.
“It didn’t happen yesterday but they are going to meet soon,” the spokesman added.
Source BBC News