By Sarah Kent 

LONDON--As the climax of Scotland's independence referendum draws near, more big companies are speaking out against splitting up the U.K. Few, though, are putting their money where their mouth is.

In contrast with the U.S., where companies often give vast sums to political parties, hardly any major businesses have made large donations to the campaign groups--Yes Scotland, which argues for independence, and Better Together, which urges a "No" vote.

Big business's main influence on the campaign has come, instead, via its warnings about the economic and employment impact of a vote for independence on Sept. 18.

Oil major BP PLC's chief executive, Bob Dudley, said this month that the future of Scotland's oil industry would be "best served by maintaining the integrity of the United Kingdom."

Banking groups Royal Bank of Scotland PLC and Lloyds Banking Group PLC have both said they would move their legal headquarters to London from Scottish capital Edinburgh following a "Yes" vote.

But the biggest donors to either side have been distinctly noncorporate. J.K. Rowling, author of the Harry Potter books, gave GBP1 million ($1.6 million) to Better Together this year. A couple who won Britain's national lottery, Colin and Christine Weir, have given Yes Scotland GBP3.5 million over the course of its campaign.

Strict U.K. rules governing campaign finance are partly the reason. Each camp may only spend GBP1.5 million during the campaign period from May 30 to Sept. 18. Better Together hit its fundraising target in early August.

Since December, both sides also have been required to disclose any donations over GBP7,500 to the U.K.'s election watchdog, the Electoral Commission. In that time, Better Together has raised GBP2.4 million from such grants, while Yes Scotland has raised GBP1.5 million, according to data published Monday.

Business leaders have been reluctant to face the public opprobrium that comes with giving large sums of money. Ms. Rowling was hit by a stream of abuse on social media following her donation.

One business leader who has made major donations is Ian Taylor, chief executive of privately owned commodities trader Vitol Group, who gave GBP500,000 to Better Together in 2013, before donations had to be declared to the Electoral Commission.

That drew heavy criticism from the Scottish National Party, the main party supporting Scottish independence. It called for the donations to be returned because of controversy around some of Vitol's historic business dealings.

"There has been a real concern about the consequences of declaring your hand for 'No', which is where big business is," said Gavin Hewitt, former chief executive of the Scottish Whisky Association. Mr. Hewitt last month helped gather the signatures of 120 business leaders for an open letter warning against Scottish independence.

Traditional British queasiness about corporate involvement in politics has also held some back.

Michelle Rodgers, a spokeswoman for Business for Scotland, a pro-independence lobby group, said it would be inappropriate for businesses to give large donations, though some individuals have given money in a personal capacity.

"The implication would be that the business was making a decision on behalf of its staff," she said. The largest donor to Business For Scotland is Brian Souter, co-founder of bus and rail operator Stagecoach Group, who donated GBP100,000, Electoral Commission data show.

"Scottish-based corporates or U.K.-based firms doing a lot of business in Scotland have been very careful not to express a view--Yes or No--fearing with some justification an adverse reaction from the Scottish government, or maybe even a boycott of their goods," said Robin McConnachie a senior adviser at consulting firm Oxford Analytica.

Write to Sarah Kent at sarah.kent@wsj.com

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