And the pact has raised fears that it could provoke China into a war.

The alliance, known as Aukus, was announced by US President Joe Biden, UK Prime Minister Boris Johnson and his Australian counterpart Scott Morrison on Wednesday.

While they did not mention China, Aukus is being widely viewed as an effort to counter Beijing's influence in the contested South China Sea.

Mr Johnson later told MPs that the agreement was "not intended to be adversarial" to China.

But the prime minister was questioned by his predecessor, Theresa May, about whether the deal could lead to Britain being dragged into war with China.

She asked the prime minister about the "implications" of the partnership in the event of a Chinese invasion of Taiwan.

Mr Johnson replied: "The United Kingdom remains determined to defend international law and that is the strong advice we would give to our friends across the world, and the strong advice that we would give to the government in Beijing."

Democratic Taiwan sees itself as a sovereign state, but Beijing has increased pressure on the island which it views as a breakaway province.

'A very low moment'

Meanwhile Washington has sought to quell anger in Paris at the pact, which has scuppered a multibillion-dollar submarine deal France had signed with Australia.

France's Foreign Minister Jean-Yves Le Drian called the announcement a "stab in the back".

He called it a "brutal, unilateral and unpredictable decision" that reminded him of former US President Donald Trump.

French diplomats in Washington cancelled a gala to celebrate ties between the US and France in retaliation.