By Simon Clark

A high-profile investor in HSBC Holdings PLC and Standard Chartered PLC has criticized the London-based banks for supporting China's plan to impose a new national security law on Hong Kong, calling on them to speak out if it results in abuse of democratic freedoms.

U.K. asset management firm Aviva Investors, a top 20 shareholder in both banks, expressed concern that the banks had backed the law despite a lack of clarity about its potential impact.

"We are uneasy at the decisions of HSBC and Standard Chartered to publicly support the proposed new national security law in Hong Kong without knowing the details of the law or how it will operate in practice," David Cumming, the chief investment officer for equities at Aviva Investors, said Tuesday.

HSBC and Standard Chartered declined to comment on Aviva's statement.

Aviva Investors is among the largest European fund managers, running GBP346 billion ($439 billion) at the end of last year.

China says the new law is necessary to restore order to Hong Kong after a year of protests. The U.K., U.S. and other governments oppose the law, saying it undermines Hong Kong's autonomy, which was agreed when the U.K. handed the city back to China in 1997.

Photos of HSBC Asia chief Peter Wong signing a petition backing the proposed security law were posted on one of the bank's social-media accounts on June 3. On the same day, a Standard Chartered representative said the law "can help maintain the long term economic and social stability of Hong Kong."

Aviva's Mr. Cumming said that the decisions made by the banks have consequences.

"If companies make political statements, they must accept the corporate responsibilities that follow," Mr. Cumming said. "Consequently, we expect both companies to confirm that they will also speak out publicly if there are any future abuses of democratic freedoms connected to this law."

HSBC publicly backed the law after being criticized by pro-Beijing figures and Chinese state media for not getting behind it. HSBC makes most of its profit in Hong Kong and mainland China. Standard Chartered is also heavily Asia-focused.

But the decisions of the banks spurred criticism from politicians in the U.K., where they are based and regulated.

"Businesses championing authoritarian powers should remember that other customers' views on human rights may not align with those of the Chinese Communist Party," Tom Tugendhat, a British lawmaker who chairs the Foreign Affairs Committee, said in an email to The Wall Street Journal.

Write to Simon Clark at simon.clark@wsj.com