Mercer left the government on Tuesday expressing frustration at a lack of progress over legislation to protect British soldiers who served in the Troubles.
Ministers have since promised to bring forward legislation to protect from historical prosecutions veterans who served in Northern Ireland, which the Tories promised in their 2019 election manifesto.
But Mercer said “the truth is that nothing has been done” as he said of ministers: “Shooting straight is not one of their finest qualities.”
The former Army officer said he thought Johnson is “deeply committed” to the manifesto promise but said the prime minister “should expect” his ministers to show the same level of responsibility.
Speaking to Times Radio, the former Army officer said: “This is the most distrustful, awful environment I’ve ever worked in, in government.
“Almost nobody tells the truth is what I’ve worked out over the last 36 hours.
“And, you know, so so I don’t think anyone really can get on their high horse about trust and ethics and all the rest of it in politics, because as far as I’m concerned, most of it is a bit of a cesspit, I think we do have a clear commitment to follow through on our promises and do right by those who serve.”
Mercer also accused the government of leaking his plans to resign and then lying about it.
“It’s pretty clear that not everyone tells the truth up here do they.
“I mean, I told people, I was resigning as a courtesy to government.
“Three hours later, it’s in the press.
“And an hour later, there’s someone following me around London with a camera.
“And, of course, they all denied they ever leaked it.
The Plymouth Moor View MP said the government was guilty of “gross betrayal of people who signed up to serve in the military” by failing to protect UK soldiers.
“The reality is that for these people, their experiences after having served this nation, 50 years later, they are constantly being dragged over to Northern Ireland, and asked to relive their experiences, it is people are drinking themselves to death. It is breaking up families, it is ruining our finest people,” he said.
“And all they did was serve at the behest of this government at the behest of the House of Commons, to uphold the rule of law on the peace in Northern Ireland.”
Mercer’s replacement Leo Docherty told the Commons on Wednesday: “A bill will soon come forward from the Northern Ireland Office that will protect our Northern Ireland veterans of Operation Banner and address the legacy of the Troubles.”
Johnson also told MPs that the government would be “bringing forward further measures in due course” when challenged by the DUP to protect Northern Ireland veterans from “vexatious” prosecutions.
Downing Street said details of the new legislation would be confirmed in the Queen’s Speech on May 11.