A string of former Prime Ministers are expected to appear at a ‘Line of Duty-style’ inquiry into Westminster’s lobbying scandal.

The Public Administration and Constitutional Affairs Committee (PACAC) is set to investigate lobbying rules amid mounting questions over David Cameron’s cosy chats and texts with ministers.

It’s thought Tony Blair, Gordon Brown and Sir John Major will be invited to give evidence before the committee.

And David Cameron’s spokesperson has already said the ex-PM would respond “positively” to any invitation to a Parliamentary committee.

According to the Telegraph, the only living former PM who will not be called to give evidence will be Theresa May, who is still a sitting MP.

Details of the committee’s probe are expected to be announced this week.

Chairman William Wragg said: “PACAC may not be the AC12 of Whitehall, nor do we envisage encountering anything quite as exciting as in a TV drama.

“However, it is at least a sense of duty that motivates our work, just as duty and service motivates the vast majority of those in public life. We must not let the questionable judgment of a few tarnish all."

The PACAC probe is one of seven inquiries the Government faces in response to the Greensill row.

The National Audit Office, which scrutinises Government spending, announced a probe into Greensill and its involvement with coronavirus support schemes.

The Treasury committee, Public Accounts Committee and the Committee on Standards in Public Life will all probe various aspects of lobbying.

David Cameron's spokesman has said he will respond "positively" to any invitation
David Cameron's spokesman has said he will respond "positively" to any invitation

The Cabinet Secretary, Simon Case ordered civil servants to urgently declare if they were moonlighting in the private sector after it emerged that a senior official had worked for Greensill while on the Government payroll.

And the Prime Minister’s own inquiry, led by Business department non-executive director Nigel Boardman, will look into David Cameron’s lobbying on behalf of Greensill - but will have no enforcement powers and isn’t likely to make any recommendations.

George Eustice, the Environment Secretary, today defended David Cameron over his cosy chats and texts with ministers including Chancellor Rishi Sunak on behalf of failed bank Greensill.

“He has not broken any of the rules. It is acceptable,” Mr Eustice said.

He also argued ministers shouldn’t have to declare who they meet with, because it doesn’t “matter”.

"Fundamentally what matters is how ministers react, not who they talk to," he said.

He argued that in his role he meets with a range of people, including constituents, farmers and environmental activists.

He said: "What matters is not who I've spoken to, but whether I'm unduly influenced by the people I speak to. And I'm not."

BBC Host Andrew Marr put to him that his argument amounted to "let us regulate ourselves, we are above and beyond suspicion."

In response, Mr Eustice argued the rules on lobbying - which allowed David Cameron's texts to Mr Sunak to go undeclared, and the "private drink" with Matt Hancock to go undeclared - were already "robust."

"What we've got," he said, "are some quite robust systems in place. And the principle one is the ministerial code, and that's how ministers conduct themselves based on the people they've talked to."

In November, the official formerly in charge of the ministerial code, Sir Alex Allen, resigned after Boris Johnson overruled his advice and refused to sack Home Secretary Priti Patel over "bullying" behaviour.

Defending former PM Mr Cameron, Mr Eustice argued former ministers cannot be "begrudged" taking up new posts following a two-year period after leaving government.

Mr Eustice, who was Mr Cameron's press secretary when he was leader of the opposition, said: "I think the key thing is that he has not broken any of the rules.

"It is acceptable, because it was within the rules.

"The point I would make is that ministers, when they leave office, including prime ministers, aren't allowed to take any such paid roles for two years - these are rules that David Cameron himself brought in.

"He left office some five years ago and you can't begrudge people moving on to another career."