Telecoms giant Vodafone has warned it could move its headquarters out of the UK following the Brexit vote.
The group - one of the UK's largest companies - says its decision would depend on whether Britain's negotiations to quit the European Union would restrict free "movement of people, capital and goods".
"The UK's membership of the European Union has been an important factor in the growth of a company such as Vodafone," a statement said.
"Freedom of movement of people, capital and goods are integral to the operation of any pan-European business."
The group said it was too early to "draw any firm conclusions regarding the long-term location for the headquarters", but added it would "take whatever decisions are appropriate in the interests of our customers, shareholders and employees".
Vodafone employs more than 13,000 people in the UK and has headquarters in London, with offices in Newbury, Berkshire.
It made 11 per cent of group profits from the UK in the last financial year, with more than half - 55 per cent - coming from its European operations.
It is the latest firm to sound the alarm over the Brexit vote and impact of leaving the single European market.
Business Secretary Sajid Javid met business groups and company executives on Tuesday to discuss the challenges and opportunities of the vote to leave the EU.
He sought to assure industry leaders that the UK is "open for business" and announced moves to calm fears about the impact of Brexit.
Javid said the biggest issue raised at the meeting was the need to secure continued access to the single market.
"My number one priority will be just that in the negotiations to come," he said.
He announced that he will be leading a series of trade missions to meet inward investors while trade minister Lord Price will be visiting countries including China and Brazil.
Business Department officials will also be visiting businesses across the UK to answer any concerns.
It is understood Vodafone's warning relates to its head office in London and not the offices in Newbury.
The group added it would be boosting its regulatory and public policy activities in Brussels to "ensure the group's substantial businesses within the European Union continue to be represented appropriately".
While it is one of the biggest groups listed on the FTSE 100 Index in London, the majority of its 462 million customers, 108,000 employees and 15,000 suppliers are based outside the UK.
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