The UK must reconsider its relations with Saudi Arabia if the state is found to have ordered the murder of the journalist Jamal Khashoggi, a senior MP has said.

Tom Tugendhat, the chair of the Commons foreign affairs committee, said the UK should work with its allies on its response to claims Khashoggi was killed during a visit to the Saudi consulate in Istanbul.

Tugendhat said it was important to establish the facts, but that if Riyadh had murdered Khashoggi or sanctioned his killing there should be a downgrading of diplomatic relations and a boycott by UK ministers.

Saudi Arabia faces a chorus of international calls to shed light on what happened to the Washington Post columnist, and business leaders have already shunned the regime.

Saudi isolation grows over Khashoggi disappearance Read more

The UN secretary general, Antonio Guterres, said he wanted to know the truth about what had happened and expressed concern that such disappearances would happen more regularly and become a “new normal”.

Lionel Barber, the editor of the Financial Times, announced that the newspaper would pull out of its partnership in a high-profile economic conference in Riyadh, and Sir Richard Branson has frozen several business links with the country.

Tugendhat said the international trade secretary, Liam Fox, should boycott the Future Investment Initiative in Riyadh later this month if Saudi involvement in 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,” Tugendhat told BBC Radio 4’s Today programme. “The idea that we can treat Saudi as a normal state if it practises state-sponsored murder outside its borders is simply not true.

“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 22 October. We will update on his activity in due course.”

Guterres told the BBC: “We need to have a strong request for the truth to be clear. We need to know exactly what has happened and we need to know exactly who is responsible and, of course, when we see the multiplication of this kind of situation I think we need to find ways in which accountability is also demanded.”

Malcolm Rifkind, a former Tory foreign secretary, called for Britain to impose sanctions. “If the current crown prince remains in power for the indefinite future, then in the first instance the United Kingdom must work with the United States, France and other countries to see if there can be a combined response, a punishment of some kind, of sanctions of some kind,” he told the BBC’s Newsnight.

“If the United States was not willing to take part then the United Kingdom has to consider action that it will take in its own name.”

A critic of Saudi Arabia’s de facto ruler, Crown Prince Mohammed bin Salman, 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 last Tuesday to obtain a document confirming he had divorced his ex-wife to allow him to remarry. Turkish officials have said he was killed on the premises and his body removed.

Saudi officials say the allegations as baseless.