Conservative MP Johnny Mercer has said the Government is "the most distrustful, awful environment" where "almost nobody tells the truth".
It comes after the former British Army officer resigned as Minister for Defence People and Veterans this week.
Mr Mercer left the post after expressing frustration at a lack of progress on legislation to protect British veterans who served during the Troubles in Northern Ireland from prosecution.
The ex-defence minister said "nothing has been done" regarding the "gross betrayal of people who signed up to serve in the military" and gave a damning account of Boris Johnson's Government during an interview with Times Radio.
"This is the most distrustful, awful environment I've ever worked in, in Government," he said.
"Almost nobody tells the truth is what I've worked out over the last 36 hours."
Mr Mercer, the MP for Plymouth Moor View, said he was "made to feel like… the last man in the room who's willing to fulfil our manifesto commitments", describing politics as a "cesspit".
"You know, I find this place has taught me a lot about the Government, a lot about my colleagues: let's say shooting straight is not one of their finest qualities," he added.
Mr Mercer had been heavily involved in the Overseas Operations Bill, which has been considered by MPs as it goes through its final stages in Parliament, with some Lords amendments rejected by the Commons.
In his resignation letter to the Prime Minister, Mr Mercer said the Government risked "damaging an already bruised veterans cohort further".
He also said he had hoped Mr Johnson's premiership would be "a step-change in veterans affairs in the UK" and had raised his issues in person during a meeting with the Prime Minister.
However, Mr Mercer said he remained "genuinely appalled" by the experiences of some British military veterans.
"I fought and bled alongside them, I've been far more fortunate than many of them since, and I have a duty to tell their truth to power," he added.
Mr Mercer said in the letter to the Prime Minister that not including those among who served during the Troubles in legislation was his "red line".
"They are not second-class veterans," he said.
"They deserve the protections of the Overseas Operations Bill like everyone else."
Mr Johnson said in his reply to Mr Mercer that he was "grateful" for his contribution as Veterans Minister and that he had "made a real difference" to the lives of defence personnel and veterans.
He said the Overseas Operations Bill is a "crucial part" of efforts to protect personnel against "vexatious and repeated" legal claims, and committed to doing more over the coming months.
Former Scots Guards captain Leo Docherty was appointed as Mr Mercer's successor on 21 April, and promised legal protections for veterans of Northern Ireland would be coming "soon".
Cover image: Johnny Mercer speaking with Union flags behind him (Picture: PA).