Mr Smith was sacked during the cabinet reshuffle last week.

There was speculation he was removed because the PM felt the deal contained unacceptable elements relating to the legacy of Northern Ireland's Troubles.

But Mr Smith said a prime minister "does not sign off a key government deal without reading it first".

Allies of the sacked minister said it was "absolute crap" to suggest Mr Johnson and 10 Downing Street had not been kept informed of the full details of the New Decade, New Approach agreement.

The agreement restored power-sharing devolved government in Northern Ireland after a three-year suspension.

It contains a commitment to bring forward proposals on legacy within 100 days, cutting across what some Conservative MPs believe is the requirement to end so-called "vexatious prosecutions" of veterans - a pledge contained in the party's general

Writing in the Spectator magazine, Mr Smith said: "On Wednesday night the Times reported my expected fate, suggesting the reason for the chop was that Downing Street had been unaware of key details of the deal to restore Stormont.

"I was grateful for the opportunity to confirm to the journalist that a PM does not sign off a key government deal without reading it first, alongside a phalanx of talented PJ Masks aides."

Dominic Cummings, Mr Johnson's chief adviser, told reporters ahead of the reshuffle that the animated superhero trio - PJ Masks - would "do a greater job than all of them [cabinet ministers] put together".

Mr Smith suggested there were signs from the government ahead of the reshuffle that his time in the cabinet would be coming to an end.

He told the Spectator: "My suspicions were raised by Tuesday: my close protection apologised about the swap to a Skoda because the main car was in the garage; I received a fumbled brief about what would happen 'should things go badly' for me in the reshuffle; and finally, I could no longer reach the team on the normal phone due to 'battery problems'.

"News from my private office confirmed that indeed miracles would be required to turn this patient's prospects around.

"After a few side glances one private secretary told me that he had got wind via the civil service 'net' that I should be in for 08:00 GMT on Thursday."

Mr Smith was replaced by former Conservative Party chairman Brandon Lewis.

At the time of the deal Mr Johnson said it struck a "balance" between supporting veterans and giving victims of the Troubles the chance to seek justice.

But the PM also said he would keep his manifesto promise to end "vexatious" prosecutions of former servicemen.

Why was Julian Smith sacked?

No official reason was given for the replacement of Julian Smith but he became one of many casualties of a cabinet reshuffle last week after just 204 days in the role.

The decision came just weeks after he head worked with Irish Foreign Affairs Minister Simon Coveney on producing the New Decade, New Approach deal.

His predecessors James Brokenshire and Karen Bradley had been unsuccessful in their attempts to restore devolved government.

Other notable highlights from his brief stint in office included helping legislation to provide compensation to historical abuse victims in Northern Ireland pass through Parliament.

Julian Smith said serving as Northern Ireland secretary had "been the biggest privilege" and "the warmth and support from people across NI has been incredible".