A former minister urged Vaughan Gething to “do the right thing” and return a £200,000 leadership campaign donation from a convicted environmental polluter.

Lee Waters, who was the deputy minister for climate change until last month, described accepting the donation from Dauson Environmental Group as unjustifiable and wrong.

He said: “£200,000 is a staggering amount, unprecedented in Welsh politics, and over four times larger than the £45,000 spending cap the Labour Party sets to ensure a fair contest.

“The fact it came from a waste company with a conviction for damaging the Gwent levels, at a time when some of us were fighting hard to protect this sensitive area, really shocked me.”

He made the comments as the Senedd debated Conservative calls for an independent investigation and Plaid Cymru proposals for a cap on donations.

He told the Senedd: “It would not be a sign of weakness to say it was a mistake to take the donation and now all the facts are known, to give it back.

“It can still be done – in my view it should be done – and sometimes doing the right thing is the hardest thing but you rarely regret it in the end.”

Mr Waters welcomed the appointment of Carwyn Jones, the former first minister, to lead an internal review of Labour’s election processes, including campaign finances.

The Llanelli MS accused the Tories of double standards, saying two home secretaries, a chancellor and a former prime minister were found to have broken the ministerial code.

He said: “What did Rishi Sunak and the Welsh Conservatives have to say about that? Not a dicky-bird. We can all see through their double standards.”

Andrew RT Davies criticised Vaughan Gething for failing to turn up at the start of the debate on his responsibilities and leaving it to another minister to respond.

He raised concerns about a £400,000 loan to a subsidiary company of Dauson from the Welsh Government-owned Development Bank in the same financial year as the donation.

Stressing that the donation was correctly declared and there was no rule-breaking, the Tory group leader said perception of a conflict of interest has caused considerable public disquiet.

Rhun ap Iorwerth described the first minister’s “nothing-to-see-here” attitude as at best complacent and at worst contemptuous towards the electorate.

“People will come to their own judgement about his absence this afternoon,” he said.

Plaid Cymru’s leader warned: “This chapter raises broader concern about the fundamental health of our democracy. There is a risk of a poison affecting our politics here.”

Jane Dodds, the Lib Dems’ leader in Wales, agreed that the episode has cast a dark shadow over Welsh democracy as she urged Mr Gething to give the money back.

She argued the wider goal should be to eradicate big money from our politics.

Adam Price, the Plaid Cymru MS for Carmarthen East and Dinefwr, said some people will perceive that the first minister's job was bought as a result of a donation in a close election.

He told the chamber: “No-one should ever be left thinking money talks, that money is able to control the future of our politics, of our democracy.”

Hefin David, the Labour MS for Caerphilly, described the row as a non-issue.

He stressed the donation was properly registered and Mark Drakeford instigated an investigation while he was first minister, finding no breach of the ministerial code.

Dr David said: “We are, for the first time, asking for a politician who’s obeyed all the rules – demonstrably obeyed all the rules – to be investigated, and I think that is frankly absurd.”

Raising concerns about hypocrisy, he highlighted six-figure donations to Plaid Cymru.

Alun Davies, a fellow Labour backbencher, who represents Blaenau Gwent, advocated introducing a fit-and-proper-persons test for political donations.

Jack Sargeant, the Labour MS for Alyn and Deeside, raised trade unions’ concerns about the potential unintended consequences of a cap on donations.

Jane Hutt, responding for the Welsh Government, stressed there is no link between the donation and any arm’s-length loan decision made by the Development Bank.

She said the first minister, who took his seat in the chamber more than an hour into the debate just before the vote, has complied with all the relevant rules.

Ms Hutt, who is chief whip and trefnydd, the Senedd’s equivalent of the leader of the house, did not say whether she would have accepted the donation when questioned.

The Conservative motion was narrowly rejected 25-27 following the debates on 1 May, while Plaid Cymru’s motion was defeated 11-27 with 14 abstensions.