LONDON — Former British Prime Minister Gordon Brown made an impassioned call Monday for Labour Party supporters to vote to stay in the European Union, amid nervousness in the “remain” camp that it is losing momentum ahead of next week’s referendum.

Brown, who governed from 2007 to 2010, says Britain should “lead in Europe” and not leave it. His speech on the EU, due later Monday, is considered significant because Brown’s last-minute intervention during the 2014 Scottish independence referendum is credited with bolstering support for remaining part of the United Kingdom.

Polls suggest the June 23 vote on whether to leave the 28-nation bloc could go either way. Labour leader Jeremy Corbyn has long criticized the EU, and some Labour members feel Corbyn has not made a strong case for staying in the bloc. That has made Conservative Prime Minister David Cameron by far the most vocal proponent of a “remain” vote — and left many Labour voters confused about where their party stands.

With just 10 days to go until the vote, senior Labour figures are stepping up their campaigning.

Labour’s foreign affairs spokesman, Hilary Benn, said in a speech Monday that Britain has “always looked beyond our own shores and engaged with the wider world,” and argued that leaving the EU “would make us a poorer Britain. A lesser Britain. A less influential Britain.”

European Council President Donald Tusk delivered an even starker warning, saying a British exit could start the unraveling of the political system that had dominated Europe for much of the past century.

Tusk told German daily Bild that Brexit “could in fact be the start of the process of destruction of not only the EU but also of the Western political civilization.”

“Not only economic implications will be negative for the U.K., but first and foremost geopolitical,” he said.” Do you know why these consequences are so dangerous? Because in the long-term they are completely unpredictable.”

He said enemies of the EU “will open a bottle of champagne” if Britain leaves.

“Leave” campaigners have accused pro-EU forces of engaging in “Project fear” because of their claims Brexit would cause economic and political turmoil.

Gisela Stuart — one of the few Labour legislators backing a “leave” vote — said the EU “only heads in one direction, seizing more power and money from member states at every opportunity.” She said leaving would let Britain “take back control of our borders, economy and democracy.”

Telecoms giant BT became the latest company to take sides Monday, sending a letter to 80,000 staff urging a “remain” vote. The letter said “the majority of businesses — large, medium and small — believe we are better off staying in.”

“Virtually all major independent experts believe leaving the EU would result in an economic downturn, one from which it may take several years to recover,” said the letter, signed by chairman Mike Rake, chief executive Gavin Patterson and leaders of two major unions at BT.

Many large private-sector employers have said they want Britain to stay in the EU, citing access to the bloc’s single market of 500 million people. Small firms are more divided, with some saying they would like to be free from EU regulation.