A former veterans minister has hit out at the government as "the most distrustful, awful environment I've ever worked in".
Tory MP Johnny Mercer told Times Radio "almost nobody tells the truth," and election pledges had not been delivered.
It follows his departure over legal protections for British troops who fought during the conflict in Northern Ireland known as the Troubles.
No 10 has pledged action on the issue.
Speaking to BBC Newsnight in an interview that will air later on Wednesday, Mr Mercer said Boris Johnson had failed to deliver on promises to protect ex-soldiers who served in Northern Ireland from prosecution.
He told the programme he had been "sacked by text" after offering to resign at the end of Wednesday, but refusing to go earlier.
On Tuesday, a No 10 spokesperson said the PM had "accepted" Mr Mercer's resignation.
Leo Docherty, who replaced Mr Mercer on Wednesday, promised legislation to offer "protection" to ex-soldiers who served during the Troubles.
They have been excluded from a bill making its way through Parliament which aims to prevent unfounded prosecutions over conflicts overseas.
The Overseas Operations Bill, which MPs debated on Wednesday, will only cover conflicts abroad such as the wars in Iraq and Afghanistan.
Mr Mercer told BBC Newsnight legislation for troops who fought in Northern Ireland had been promised repeatedly but "has never appeared".
"The government's been promising for a long time this bill. If I thought there was any prospect of that bill coming, I clearly would have stayed," he added.
The Plymouth Moor View MP said he had asked officials before his departure whether a "single word had been committed to paper" and had been told "no".
Speaking to Times Radio, he said "nothing has been done" over the issue of Northern Ireland veterans, despite previous Conservative commitments.
"Politics is a question of integrity, this whole trust thing," he added.
"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.
"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".
On Wednesday, the PM's official spokesman said details new legislation for Northern Ireland would be announced "in the coming weeks".
"There is more to be done on the issue of the Northern Ireland legacy and we are committed to making progress on this as quickly as possible," he added.
"We are engaging with Northern Ireland parties and the community in Northern Ireland, including victims' groups as well as the Irish government, on the way forward."