Donald Trump "is f***ed" after his former campaign chief was charged as part of an investigation into Russian interference in last year's election, an ex-aide has said.

Sam Nunberg, an adviser to the Republican's presidential campaign, said Paul Manafort's indictment showed special prosecutor Robert Mueller would "go over every financial dealing" of Trump's businesses and allies. 

He suggested the President may not be able to survive the probe alongside dwindling approval ratings.

Mr Manafort and business associate Rick Gates are under house arrest after pleading not guilty to 12 charges including money laundering and conspiracy to defraud the US. 

As part of the same investigation, Mr Trump's former foreign policy adviser George Papadopoulos has admitted making false statements to the FBI about his contact with the Russian government.

“Here’s what Manafort’s indictment tells me: Mueller is going to go over every financial dealing of [senior Trump adviser and son-in-law] Jared Kushner and the Trump Organisation," Mr Nunberg told Vanity Fair.

Referencing the President's worst approval rating since his election, he added: "Trump is at 33 per cent in Gallup. You can’t go any lower. He’s f***ed.”

The results of Gallup's latest tracking poll were gathered before it had emerged Mr Mueller's team had filed indictments against three of his former campaign staff. The survey found 63 per cent of Americans were unhappy with the President's performance.

Mr Nunberg, a Republican political analyst, described Mr Kushner as "the worst political adviser in the White House in modern history".

Mr Trump is reported to blame Mr Kushner for his role in decisions, such as the firing of FBI director James Comey, that led to the Russia probe.

Mr Nunberg worked as an adviser to Mr Trump's presidential campaign until August 2015, when he was fired after allegedly posting racist comments on Facebook.

He was later accused of breaching a confidentiality agreement by leaking a story about tensions within the campaign. He claimed the allegation was an attempt to "silence" him after he endorsed Ted Cruz.

Mr Trump sued for $10m (£7.5m) but the dispute was settled out of court.