On Sunday, claims emerged that the US President had presented the German Chancellor with an invoice for an undisclosed sum – believed to be around £300bn.

The bill – which was said to include how much German spending fell below the two per cent Nato target over the past 12 years, plus interest - was reportedly given to Ms Merkel during talks held in Washington earlier in March.

But White House press secretary Sean Spicer attempted to pour cold water on the rumour when questioned on Sunday.

"No, this is not true," he said. Michael Short, a White House spokesperson, also said the report was "false”.

While solid proof of the bill has yet to emerge, the allegation appears to fit with Mr Trump’s long-standing criticism of countries he says are not paying their fair share of the military alliance’s budget.

The unorthodox move would also suggest relations between the two countries are even more strained than previously thought.

Donald Trump apparently snubs handshake with Angela Merkel

At a joint press conference after the 17 March meeting in Washington, Mr Trump said he had “reiterated to Chancellor Merkel my strong support for NATO, as well as the need for our NATO allies to pay their fair share for the cost of defence.

“Many nations owe vast sums of money from past years, and it is very unfair to the United States. These nations must pay what they owe.”

Ms Merkel replied that Germany was “committed to this two per cent goal until 2024” and said her government was “going to work together again and again on this”.

The President later tweeted that Berlin owed the US and Nato “vast sums of money” for the “powerful, and very expensive, defence it provides to Germany”.

The incident involving the alleged invoice first came to light in a report in The Sunday Times, which quoted an unnamed German government minister as saying the move was “outrageous”.

“The concept behind putting out such demands is to intimidate the other side, but the chancellor took it calmly and will not respond to such provocation,” the minister said.