The UK ambassador to the EU should have resigned when the result of June's referendum was announced, UKIP Wales MEP Nathan Gill has said.
Sir Ivan Rogers urged Brussels colleagues to challenge "muddled thinking and... speak truth to power" as he quit ahead of Brexit talks.
Mr Gill said the resignations allowed a "pro-Brexit ambassador" to be appointed for the crucial negotiations.
The first minister's spokesman called the resignation "deeply concerning".
Writing to staff, in a letter obtained by the BBC, Sir Ivan said UK ministers needed to hear "unvarnished" and "uncomfortable" views from Europe.
The government said Sir Ivan had quit earlier so a successor could be in place before Brexit negotiations started.
Sir Ivan's note confirmed this but also warned "the government will only achieve the best for the country if it harnesses the best experience we have".
Mr Gill told BBC Wales on Tuesday that he was "glad that Sir Ivan Rogers has gone" and "we can now have a pro-Brexit ambassador in place for the renegotiations which will be happening this year".
"The people of Wales and the UK spoke very clearly in the referendum on June the 23rd - they wanted out," he said.
"Personally I think Sir Ivan should have resigned once that Brexit result was declared."
The resignation came after Sir Ivan's deputy in Brussels, Shan Morgan, announced in November that she would be leaving the post to become the Welsh government's top civil servant, a job she is due to start within weeks.
In his letter, Sir Ivan said Ms Morgan would be "hugely missed" and would be a "tremendous asset to the Welsh Government".
A spokesman for First Minister Carwyn Jones said Sir Ivan's departure "at a time of such uncertainty" was "deeply concerning".
"As the negotiations on the UK leaving the European Union near, we need clarity and stability in our approach," the spokesman said.
"It is vital that a successor is appointed immediately to take up this crucial role."
Labour MEP Derek Vaughan described Sir Ivan as a "competent, well connected diplomat who puts the interests of the UK first".
"I believe he was frustrated with the muddled thinking of government ministers who would not accept the message from the EU passed on by Sir Ivan that full access to the single market means accepting freedom of movement [of labour]," he said.
"Hopefully, other politicians and trade union leaders will accept this and prioritise access to the single market."