London | The world's two largest mining companies have urged Britain to remain part of the European Union, arguing that a "Brexit" could damage the global economy and make London a less attractive destination for Australian businesses.
BHP Billiton and Rio Tinto, which are both listed on the Australian and London stock exchanges, argue that the UK's historical position as a "gateway to Europe" for Australian corporations would be threatened if the UK votes to leave the 28-nation bloc. 
With the Remain camp holding a slender one-point lead according to an online opinion poll by YouGov released on Friday, BHP chief executive Andrew Mackenzie told The Australian Financial Review he "strongly believed" Britain should stay as part of Europe "for the sake of Britain, for the sake of Europe and actually for the sake of the world".
"There are many imperfections, as there always are, with large blocs like the EU, but on balance I think it exists as a force for good," he said ahead of the   June 23 referendum.
Australian officials, such as Foreign Minister Julie Bishop and the High Commissioner to the UK, Alexander Downer, have argued that having a strong UK as part of Europe was in Australia's best interests. They believe having a country with shared political and economic values within the world's largest single market is particularly advantageous with formal negotiations for a free trade agreement between Australia and the EU due to begin in early 2017.
Mr Mackenzie, who was born in Scotland and has worked in Germany, Norway and France, said "losing this gateway [to Europe] I think would be bad and regrettable for Australia". While a vote to leave the EU would not prompt significant changes at BHP, Mr Mackenzie said it could make a difference for new companies looking to set up in Britain.
"If you are a new company that is thinking of listing in London, you might say: 'Hold on, if I list in London I'm not going to get as connected to Europe'. So you might think twice about listing in London."
Campaigning in the referendum, which has focused on the economy and immigration, officially begun on Friday with polls suggesting as much of 20 per cent of voters are yet to make up their minds.
Rio chairman Jan du Plessis told the Financial Review that his personal view was that a "Brexit" would cause Australian, Canadian, and South African companies that have offices in London for "cultural, legal and other practical reasons" to have "second thoughts".
"It's just obvious," he said. "I'm not predicting that London is going to die. This is a robust city. But I think the UK will suffer because many, many people - and not just from the English-speaking world - are comfortable living in the UK ... If this is chopped off from the rest of Europe this will be put at risk."
The South African-born Mr du Plessis, who has lived in Europe since 1981 and holds a British passport, said that despite the EU's "considerable imperfections", he believed the "benefits of membership vastly outweigh the risks and disadvantages of no longer being in that club".
Larger businesses have generally supported the Remain camp although many smaller firms share broader community concerns about the EU capital of Brussels lacking proper accountability and generating burdensome regulation.
Mr Mackenzie said he sympathised with frustrations about EU bureaucracy, but argued that the Brussels did a good job balancing "social progress" policies with the need for free markets and trade.
"Some of the regulation they [anti-EU business leaders] want to resist are the regulations of progress so there is a regressive nature to saying 'Leave me alone, I don't want to constrained about the gender diversity of my company or working hours or how I protect people with health and safety'. And I say if we don't create this level playing field we will have a race to the bottom and ultimately it will be incredibly bad for business."
Both Mr Mackenzie and du Plessis deplored the negativity that characterised both sides of the debate and argued that a more positive case for the EU needed to made. Pointing to the "70 years of peace" Europe has enjoyed since integration began after the Second World War, Mr du Plessis said "the big problem people like me face is that is much easier to make base emotional arguments about why we should leave the EU" while "the reasons for staying are rather more noble and therefore become harder to argue."