Welsh Secretary Simon Hart has become the latest cabinet member to resign as pressure continues to mount on Prime Minister Boris Johnson.
He was among a group of UK ministers who earlier urged Mr Johnson to quit.
In his resignation letter, Mr Hart said colleagues had done their utmost to "turn the ship around" but he no longer thought that was possible.
Mr Johnson has defied calls to resign, as he tries to face down a growing mutiny among his cabinet.
In a letter addressed to Mr Johnson and handed to the chief whip, Mr Hart insisted he had "desperately hoped" not to have to write it but saw "no other option".
He said Mr Johnson would be remembered as a prime minister "with energy, vision, determination and humour".
"There was never a dull moment in your government and I will be forever grateful to have been given the chance to be part of it," he said.
In his letter, he also said he had "never been a massive fan of ministerial resignations being the best way of forcing change".
Mr Hart is the third cabinet minister to step down after Chancellor Rishi Sunak and Health Secretary Sajid Javid quit within minutes of one another on Tuesday.
As of Wednesday night, 16 ministers had resigned, along with 20 government aides and five others.
Levelling Up Secretary and former leadership rival to Mr Johnson, Michael Gove, was sacked by the prime minister after being part of the group of senior Tories urging him to go.
Home Secretary Priti Patel, a former close ally, has also joined the group of rebel ministers.
Conservative Senedd leader, Andrew RT Davies, called on the prime minister to "reflect on his position and allow someone else to take on that mantle to deliver the Conservative manifesto".
It follows Mr Johnson's decision to appoint Chris Pincher as deputy chief whip earlier this year, despite being aware of misconduct allegations against him.
Who is Simon Hart?
- Came to Parliament in 2010 with a background in rural affairs as chief executive of the Countryside Alliance and a former master of the South Pembrokeshire Hunt
- The Carmarthen West and South Pembrokeshire MP was previously a junior minister in the Cabinet Office
- He was made a minister after Mr Johnson's December 2019 general election victory
During Wednesday's Welsh Questions, Mr Hart was asked by Plaid Cymru MP, Liz Saville Roberts, "when will he be going?"
In his response, he insisted it was "business as usual in the Wales Office".
One of Mr Hart's predecessors, Preseli Pembrokeshire MP Stephen Crabb, has called for Mr Johnson to resign "for the good of the country".
He said trust in the UK government "had been draining away month after month" as controversies piled up.
Mr Crabb, who confirmed he voted against Mr Johnson in the confidence vote last month, wrote in an article for the Pembrokeshire Herald: "Every day I open my email inbox, every Q&A session I hold in the constituency, every time I visit Tesco or Morrisons in Haverfordwest, the feedback I receive is consistent and overwhelming: Boris Johnson should resign."
Mr Hart's resignation came after three other Welsh Conservative MPs quit their roles as government aides, with a fourth threatening to follow suit.
Montgomeryshire MP Craig Williams and Vale of Clwyd MP James Davies have both quit.
Mr Williams, a PPS to the Treasury, said it was becoming impossible for Mr Johnson to rebuild trust with the public, while Mr Davies added that it was "clear that the party and the country are no longer governable" under him.
On Tuesday night, Virginia Crosbie became the first Welsh MP to call publicly for Mr Johnson to go as she resigned from her post in the Wales Office.
During Prime Minister's Questions, Ms Saville Roberts accused Mr Johnson of putting "political survival over public duty".
But she said whatever happened "the same Westminster arrogance will continue to dictate our futures in Wales", asking him if he wanted a "medal for being the best recruiting sergeant for independence we could wish for?"
The prime minister responded: "I see the bonds of our union are being strengthened the whole time."
Wales' First Minister Mark Drakeford called for a general election to determine which political party should occupy Downing Street.
"Let the people of the United Kingdom decide whether this is a government they want to see continue or whether, as I believe they would want, to see a fresh start," he said.