Amid the hue and cry over President Trump's travel ban, news of another potential change to American immigration rules has set off a panic attack in India's tech industry.

Major Indian tech shares took a nosedive on Tuesday on reports that Trump is planning to make changes to the H-1B visa program that allows skilled foreigners to work in the U.S.

Shares in Tata Consultancy Services (TCS), India's biggest private sector employer, plunged more than 5% on Mumbai's stock exchange, while other top firms like Infosys (INFY) and Wipro (WIT) fell by more than 4%.

TCS and Infosys declined to comment. Wipro did not respond to requests for comment.

India's vast outsourcing industry employs millions of people. Its business in the U.S., where it provides engineering and other tech services to firms such as IBM (IBM), Microsoft (MSFT) and Citibank (C), is highly dependent on the H-1B visa.

The U.S. is worth about $65 billion to India's tech industry. Indian media warned of a major setback ahead.

"[It] is a fact that these categories of workers are in short supply in the U.S.," said R Chandrashekhar, president of Nasscom, which represents India's software industry. Restricting H-1B visas would "have implications for both Indian companies and American corporations as well," he told CNNMoney.

The former U.S. ambassador to India, Richard Verma, estimated last year that 70% of the 85,000 H-1B visas issued annually go to Indian workers. The visas, which are currently allocated by a lottery system, are hugely oversubscribed -- demand for them in 2016 was three times more than the number available.

Related: Bipartisan bill aims to reform H1-B visa system

Now, that system is under attack on multiple fronts. Three separate bills have been introduced in the U.S. Congress this month aiming to curb the program in various ways.

It is also in Trump's crosshairs. A draft executive order obtained by CNNMoney calls for several changes to U.S. immigration rules, although it was not immediately clear what the specific impact on the H-1B visas would be.

"I think with respect to H-1Bs and other visas, it's part of a larger immigration reform effort that the president will continue to talk about," Trump's press secretary Sean Spicer told reporters on Monday.

Spicer added that Trump will do so "through executive order and through working with Congress."

Silicon Valley lawmaker Zoe Lofgren introduced a bill a week ago, calls for removing the lottery system and replacing it with a preference for companies that can pay the highest salaries.

That would make it far more costly for Indian firms to send their workers to the U.S.

"Cost was the major advantage for Indian companies, and that will get impacted," said D.D. Mishra, a research director at technology consultancy Gartner.

Mishra estimates that up to 30% of employees working on software projects for Indian firms are overseas. The proportion is much higher for hardware projects, meaning companies would struggle if they faced restrictions on the number of Indians they could employ.

"There is very little room to maneuver...without disrupting the quality of work," he said.

The Indian government says it has made its view known to Trump and U.S. lawmakers.

"India's interests and concerns have been conveyed both to the U.S. Administration and the U.S. Congress at senior levels," foreign ministry spokesman Vikas Swarup said Tuesday.

Related: Silicon Valley lawmaker introduces H1-B reform bill

-- Sugam Pokharel and Medhavi Arora contributed reporting