The UK is "playing catch-up" with Donald Trump over a missing Saudi journalist, shadow foreign secretary Emily Thornberry has warned.

Reports claim Washington Post columnist Jamal Khashoggi was killed while visiting the Saudi consulate in Istanbul, with Turkish authorities reported to have obtained audio and video recordings of the alleged murder.

Ms Thornberry criticised the UK response from Foreign Secretary Jeremy Hunt, saying he had failed to hold the Saudis to account on the Andrew Marr Show in the face of mounting evidence.

She pointed out how her former counterpart Boris Johnson had condemned the Russians "within hours" over what turned out to be a fake murder of Russian journalist Arkady Babchenko.

In contrast, the Labour MP said, there had been "nothing but pusillanimous mutterings" from Jeremy Hunt over the disappearance of Saudi critic Mr Khasoggi - despite the US president threatening "severe punishment".

"He is being outdone by Donald Trump, whoever would have thought that?" she said.

"They're playing catch-up with Donald Trump when it comes to condemnation of this and actually taking action and being prepared to stand firm."

Ms Thornberry said the alleged murder was "yet more evidence of how the Saudi prince in particular appears to be out of control" and "this country has had enough".

Cabinet minister Matt Hancock later told the Andrew Marr Show: "We don't know the full details yet. If the allegations are true there will be serious consequences.

"That's what the Foreign Secretary has said, he has spoken to his Saudi opposite number and he has made that clear."

Ms Thornberry also put her criticism in writing in a piece for the Observer, where she said: "We must apply the same standards to countries such as Saudi Arabia, Israel and Egypt that we apply to Iran, Russia and Syria.

"Where any of them abuse human rights and breach international humanitarian law, we must be prepared to call it out in the same measure, rather than treating it as one rule for our supposed friends and another for our supposed enemies."

Mr Hunt has been under increasing pressure to take tough action over the disappearance of Mr Khashoggi, who has not been seen since October 2, and has said that if reports of Mr Khashoggi's death prove correct, the UK would regard the situation as "serious".

Ms Thornberry said this was "too little, far too late".

In response to her article, Mr Hunt tweeted: "From the party whose leader wouldn't even condemn Russia by name after the first ever chemical weapons attack on British soil... we have been robust and will continue to be on this very troubling issue."

Riyadh has faced a chorus of international calls to shed light on what happened to the journalist, and business leaders have already shunned the regime.

In a sign of the international concern, UN Secretary General Antonio Guterres said he wanted to know the truth about what happened and expressed fears such disappearances would happen more regularly and become a "new normal".

Financial Times editor Lionel Barber announced the newspaper would be pulling out of its partnership in a high-profile economic conference in Riyadh, while Sir Richard Branson has frozen several business links with the Gulf state.

Commons Foreign Affairs Committee chairman Tom Tugendhat said on Saturday that the UK should work with its allies on its response, and that International Trade Secretary Liam Fox should boycott the Future Investment Initiative in Riyadh later this month if Saudi involvement in Mr Khashoggi's disappearance was proved.

"The first thing for us to do is for us to get together with our allies, the United States, the Europeans and others, to discuss very seriously what's going on," Mr Tugendhat told BBC Radio 4's Today.

"The idea that we can treat Saudi as a normal state if it practises state-sponsored murder outside its borders is simply not true."

He added: "We may be talking about downgrading diplomatic relations, we may be talking about restricting support for certain areas."

The Department for International Trade said: "The Secretary of State's diary is yet to be finalised for the week of October 22, we will update on his activity in due course."

A critic of Saudi Arabia's de facto ruler Crown Prince Mohammed bin Salman, Mr Khashoggi was living in self-imposed exile in the US and writing opinion pieces for the Washington Post before he vanished.

He visited the consulate on October 2 to obtain a document confirming he had divorced his ex-wife, in order to allow him to remarry.

Turkish officials have said he was killed on the premises and his body removed.

Saudi officials have denied the allegations as baseless.