Committee chair Bernard Jenkin has accused one of David Cameron's special advisers of being "clearly in breach" of official guidelines in the run-up to the EU referendum.
Special advisers – known as Spads – work in government departments as the personal appointees of ministers, giving them political, policy and presentational advice which goes beyond the remit of impartial civil servants.
But, under the Spad code of conduct, they "must not take public part in political controversy, through any form of statement whether in speeches or letters to the press, or in books, social media, articles or leaflets".
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The code states: "They must observe discretion and express comment with moderation, avoiding personal attacks, and would not normally speak in public for their minister or the department."
Jenkin – the eurosceptic Tory MP who chairs the Public Administration and Constitutional Affairs Committee (Pacac) – has now written to the prime minister saying that he belives Daniel Korski, Cameron's adviser since June 2013, has over-stepped the mark by "issuing a stream of Twitter messages to the public at large" which are in favour of Britain staying in the EU.
"Many are politically controversial," Jenkin writes. "In particular, it is completely contrary to the Code that Mr Korski re-tweeted a tweet that included an image of the secretary of state for work and pensions that was captioned 'If you're whingeing – you're losing.'"
The committee chair asks Cameron to outline what action he will be taking against Korski, saying that unless the issue "can be resolved without delay" he will summon the special adviser before MPs to explain his actions.
CSW understands that Jenkin has yet to receive a response to his letter. A Number 10 spokesperson has said: "We have received the letter and will respond in due course."
Bernard Jenkin letter to David Cameron
Separately, Jenkin's office has also tabled a series of written questions in the Commons and submitted a Freedom of Information request directly to cabinet secretary Sir Jeremy Heywood in a bid to obtain any correspondence between Number 10 officials and advisers and the former British Chambers of Commerce director John Longworth, who was suspended from his role at the head of the business lobby group over the weekend after expressing pro-Brexit views.
The BCC's official position is in favour of the UK remaining in the EU. Number 10 has said "no pressure was applied" to Longworth, while the BCC itself has said that no politician or interest group "had any influence on the BCC board decision to suspend" him.
It is not the first time that Jenkin's committee has questioned the conduct of a special adviser.
In 2014 Pacac – then called the Public Administration Select Committee – threatened to summon deputy prime minister Nick Clegg for a public grilling after querying whether his own Spad, Daniel Coetzee, had been involved in political activity.
Coetzee later switched roles, moving from the Deputy Prime Minister’s Office to Lib Dem HQ in Great George Street as the party’s general election director of strategy.
The conduct of Spads, civil servants and ministers during the EU referendum campaign has been the subject of some controversy in recent weeks, with eurosceptics arguing that curbs on the support that Cabinet ministers in favour of leaving the EU can receive puts their side at a disadvantage.
A Q&A prepared by the Cabinet Office for use during the referendum campaign says that while ministers "can decide how to spend their own time and will be accountable publicly for their decisions", their spads cannot campaign "except in their own time and outside office hours unless they wish to resign".
Cabinet secretary Jeremy Heywood told Jenkin's committee earlier this month that the civil service "would be literally tying itself up in knots" without the rules put in place to govern the support officials can give to ministers who want to leave the EU.
UPDATE: An earlier version of this story mistakenly referred to Daniel Korski as Jim. Apologies for the error - Matt
Bernard Jenkin to Jeremy Heywood - 08-Mar 2016 (1)