A pallbearer at the funeral for a Labour MS whose death has triggered a by-election has criticised Reform after he received a leaflet from the party on the day of the service.
Nigel Farage's party is among those expected to contest a by-election in Caerphilly to fill a seat left vacant after Hefin David's death.
David Bezzina said the mailout showed a lack of "common decency and respect".
A Reform source apologised if the mailout had caused upset and said it was not intended to arrive that day.
Reform is expected to contest the Caerphilly by-election, alongside all other major parties in Wales.
Plaid Cymru has announced that Lindsay Whittle is standing for the party. Labour is expected to announce its candidate at the weekend, while the Conservatives said they will begin the process of finding a candidate shortly.
Hundreds of mourners including senior politicians from across Wales' political spectrum attended the funeral in Gelligaer's St Catwg's Church on Monday.
After the funeral it emerged that a mailout had been received by residents in the constituency from Reform UK on the day of the funeral.
The leaflet in English and Welsh came in the form of a letter from Nigel Farage, arguing that Labour had failed Wales on jobs, the NHS and immigration.
It asks voters for a donation and for their contact details, with a short questionnaire on whether they would support the party, and whether they would be willing to put up a poster.
One version of the mailout had an envelope to return the form to an address in Caerphilly town centre.
Among the mourners was family friend of Mr David, Mr Bezzina, who said said he found the letter after he returned to his Hengoed home afterwards.
Mr Bezzina, who works for Labour MP Ruth Jones, told BBC Wales: "I was distressed. My wife was very close to Hefin as well - he was like an older brother to her. She was quite distressed when she saw an election leaflet on our doorstep.
"I think it just shows a total lack of respect and decency.
"I think the normal practice is that you would not engage in any campaigning activity or deliver leaflets or media statements on election campaigning until after the funeral at the very earliest."
Richard John, group leader for the Conservatives on Monmouthshire council, had also learned about the mailout, saying he was told a resident in the constituency had received it on Monday morning.
He said: "It is incredibly insensitive that on the very day Caerphilly came together to pay its respects to Hefin David, residents were receiving political mail from Reform UK asking for poster sites and donations.
"While every other mainstream party has shown respect by pausing campaigning, Reform UK appears to have chosen to press ahead with a mass mailout."
BBC Wales was told that the mailout was sent via post. A Reform source said: "We apologise if it has upset anyone. It was not intentional at all. Some have landed at people's homes after the funeral.
"But obviously there is a by-election now and we are going to fight with everything we've got to win the seat."