A former Scots Guards captain has been appointed as the new veterans minister following Johnny Mercer’s exit.

Leo Docherty replaces Mr Mercer, who left Boris Johnson’s Government after expressing frustration at a lack of progress over legislation to protect British veterans who served during the Troubles.

Former Army officer Mr Mercer said he was “sorry to have been relieved of my responsibilities in Government” amid reports that he was sacked by Mr Johnson.

In his formal exchange of letters with the Prime Minister, Mr Mercer said he was “forced” to offer his resignation with a “heavy heart”.

Defence Secretary Ben Wallace said he is “delighted” to welcome Mr Docherty to the department.

“He will be taking up one of the most important roles, championing our veterans and service personnel,” Mr Wallace said.

“Leo comes with a wealth of experience both of the armed forces, having served in Afghanistan, and of politics.

“I know that he will do an excellent job.”

Mr Mercer, MP for Plymouth Moor View, has been heavily involved in the Overseas Operations (Service Personnel and Veterans) Bill, which was being considered by MPs on Wednesday as it goes through its final stages in Parliament.

The legislation was developed in response to legal claims made after operations in Iraq and Afghanistan, but does not cover incidents in Northern Ireland.

In his letter to the Prime Minister, Mr Mercer said the Government risks “damaging an already bruised veterans cohort further”.

He said he had hoped Mr Johnson’s premiership would “signal a step change in veterans affairs in the UK” and that he had raised his concerns in a face-to-face meeting with him.

“I am, of course, desperately sad events have transpired the way they have – I truly have exhausted my efforts and my team to make it work,” he said.

“But the truth is politics always was a means to change how this country treats her military veterans, and I remain genuinely appalled by the experiences of some of this nation’s finest people who have served in the armed forces.

“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.”

Mr Mercer said in the letter to the Prime Minister that not including those who served during the Troubles was his “red line”, adding: “I am deeply proud of my predecessors who served in Northern Ireland.

“They are not second-class veterans. They deserve the protections of the Overseas Operations Bill like everyone else.

“I made promises on your behalf that we would not leave them behind and would walk through simultaneous legislation for them. No discernible efforts have been made to do so, and I can see no prospect of this changing.

“I have no choice but to leave Government and campaign for them in Parliament.”

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.

“But we are committed to doing more over the coming months, including for those who have served in Northern Ireland,” he said.

Downing Street said the Queen’s Speech on May 11 would “reconfirm our intention to bring forward the legislation on this issue”.

Meanwhile, Alan Mak was made an unpaid Government whip, filling Mr Docherty’s former role.