Donald Trump has escalated his attacks on the FBI’s investigation of Russian interference into the US presidential election, and the president’s evident frustration with the probe being led by special counsel Robert Mueller is beginning to have an effect.

Reports surfaced of an FBI informant being in contact with several Trump campaign officials in 2016, and Mr Trump has seized on the reports to claim, without providing evidence, that the FBI planted a spy within his campaign. He issued several calls for the Department of Justice (DoJ) to investigate. 

In a fresh morning salvo via Twitter, Mr Trump – who is no stranger to hyperbole – said what he called “SPYGATE” could be “one of the biggest political scandals in history”.

The president has consistently labelled Mr Mueller’s investigation – which is also looking into possible collusion between Trump campaign officials and Russia – as a “witch hunt”, insisting there was “no collusion”.

Such pressure seems to have finally had an impact, with a meeting to be held at the White House on Thursday between top government officials and two Republican legislators to allow members of Congress to review classified information relating to the spying claims. No Democrats have been invited to the meeting.

With the DoJ having asked the inspector general to widen an already open investigation into the conduct of the FBI in order to include the spy claims, Mr Trump suggested the tables had now been turned on those investigating his campaign, writing: ”What goes around, comes around!”

In his number of tweets, Mr Trump referred to those investigating him as the “Criminal Deep State,” a term that has become popular with Mr Trump and his supporters to describe long-serving unelected officials whom they claim are out to undermine his presidency.

Mr Trump also quoted Andrew Napolitano, a former New Jersey Superior Court judge, saying, “It’s clear that they had eyes and ears all over the Trump Campaign.”

Mr Napolitano is a frequent contributor to Fox News, a channel the president watches regularly, and had in fact appeared on the programme Fox & Friends earlier on Wednesday morning.

According to reports, the FBI source Mr Trump is referring to is Stefan Halper, a former University of Cambridge professor and long-time Republican. There has been no evidence yet to suggest that a source had been inserted into the Trump campaign, but it is believed that Mr Halper did have contact with at least three campaign advisers.

In an interview on Tuesday, former director of the national intelligence James Clapper said that the FBI had never spied on the Trump campaign.

“They were not,” he said during an appearance on ABC’s The View  to promote a new book. “They were spying – a term I don’t particularly like – on what the Russians were doing,” Mr Clapper added.

The FBI, according to Mr Clapper, were trying to answer the question, “Were the Russians infiltrating [the campaign], trying to gain access, trying to gain leverage and influence?”

He was backed-up by former FBI director James Comey, who was originally in charge of the investigation into Russian interference until he was fired by Mr Trump last May.

The now-frequent critic of the president tweeted: “Facts matter. The FBI’s use of Confidential Human Sources (the actual term) is tightly regulated and essential to protecting the country. Attacks on the FBI and lying about its work will do lasting damage to our country. How will Republicans explain this to their grandchildren?”

During his morning string of tweets, Mr Trump hit back at Mr Clapper’s comments, including that the president should have been grateful for the FBI doing its job. 

“No, James R Clapper Jr, I am not happy,” Mr Trump wrote. “Spying on a campaign would be illegal, and a scandal to boot!” 

Outside the White House later, Mr Trump spoke about Thursday’s meetings. “When they look at the documents, I think people are going to see a lot of bad things happen, ” Mr Trump claimed. “I hope it’s not so, because if it is, there’s never been anything like it in the history of our country… But I hope it’s not true, but it looks like it is.”

 Also on Wednesday, Mr Trump’s former campaign chairman Paul Manafort asked a federal judge to suppress evidence seized by FBI agents working with Mr Mueller as part of the case against him, saying the raids violated the US Constitution’s ban on unreasonable searches and seizures.

Mr Manafort faces charges including conspiring to launder money, conspiring to defraud the US and failing to register as a foreign agent. The charges relate to lobbying work Mr Manafort carried out for a pro-Russian former Ukrainian president before serving as Mr Trump’s campaign chairman in 2016. He has pleaded not guilty.

Mr Trump and his supporters have suggested that the case against Mr Manafort shows that Mr Mueller is overreaching in his Russia investigation. The hearing before US District Judge Amy Berman Jackson marks Mr Manafort’s latest bid to hinder the criminal case against him, though last week Judge Jackson refused to dismiss the charges.

At the hearing in Washington, Mr Manafort’s lawyers told Judge Jackson that FBI agents unlawfully conducted an initial warrantless search of a storage locker housing documents from his consulting company by improperly getting a low-level staffer to unlock it and let a special agent look around.

They also plan to challenge the legality of an FBI raid on Mr Manafort’s Virginia home, saying agents conducted an overly broad search by seizing “every electronic and media device” there. 

Mr Mueller’s office has defended the legality of the FBI raids, saying that the agents had secured written consent from the storage unit’s lease-holder and that the warrant used to search Mr Manafort’s home was not overly broad.

Reuters contributed to this report