TOKYO -- Japanese companies have signaled that British proposals for withdrawal from the EU do not resolve uncertainty over their future investment in the U.K., as spiraling crisis over the plan threatens to unseat Theresa May as prime minister.

Many told the Nikkei Asian Review that the scheme did little to address concerns they have long expressed, such as potential delays at the border for goods and people moving between the EU and the U.K. and the terms of cross-border trade.

Toyota said the plan was not detailed enough to assuage its concerns over the costs of manufacturing in the U.K. post Brexit. "Investment depends on the tariffs," a Toyota spokesman said.

Nissan echoed Toyota's concerns over the future trading relationship, which could take years to hammer out. The current proposal allows the U.K. to stay in the customs union and single market until a trade deal is negotiated.

"The U.K. has not yet announced clear a direction for [trading with the EU] after Brexit, including tariffs," a spokesman for Japanese carmaker Nissan said. Nissan runs one of Britain's biggest car plants in Sunderland, in the northeast of England. While the factory would remain its European base after Brexit, the company said it still needed details of trade between the U.K. and the EU.

Nomura said it had been "preparing for various scenarios" regardless of the deal. It would "take steps to make sure that its products and services will be provided for all current clients without disruption after the Brexit," a spokesperson told Nikkei. The financial services group has recently created a new company in Germany, and in June it was granted a securities trading license.

Japan is one of the U.K.'s largest foreign direct investors and its companies have long seen Britain as a gateway to Europe's 500 million consumers. Since Britain voted to leave the EU in 2016, several companies have announced plans to shift operations or headquarters to Europe. The question of Britain's future relationship with the EU is considered such a priority that Prime Minister Shinzo Abe voiced concerns about the lack of clarity during his visit to the U.K. in October.

U.K. Business Secretary Greg Clark was in Tokyo last week to reassure Japanese companies that the government's draft proposals would address their concerns.

But Toyota told Nikkei that it could still be forced to halt production temporarily at its plant in Burnaston, near Derby in the Midlands of England, even with the proposals published this week. The company is concerned that its just-in-time manufacturing system could be held up by delays at the border after Brexit, the spokesman said. "We buy auto parts from all around Europe and do not have inventories." The company produced 144,000 vehicles in the U.K., roughly 80% of which are exported to EU countries.

Jun Washizawa, a deputy director with the Japan External Trade Organization, said the proposals "did not mark a stage that changes the outlook. The important point is whether the U.K. government can pass the draft in the Parliament or not."

But the risk that will not happen is increasing.

The draft proposals, approved by the cabinet on Wednesday after a marathon five hour meeting, sought to avoid "no-deal Brexit," by keeping the U.K. in the EU customs union indefinitely until it can strike a trade deal with the EU and resolve the difficult issue of the border between Northern Ireland and the Republic. Almost immediately, however, ministers began to resign from the government, sparking a leadership crisis that is expected to intensify on Friday.

The prime minister has argued that the proposals represent the best deal possible after 20 months of negotiations.

That view received support from some Asian companies. Honda welcomed the scheme. "It is favorable for Honda that Brexit was approved at the cabinet," a spokesperson for the Japanese carmaker commented. The company has a manufacturing plant in Swindon, where its Civic Hatchback is produced.

William Cheng Kai-man, chairman at Hong Kong-listed Magnificent Real Estate, remained bullish over London's hotel market and is actively looking to buy more hotels. Cheng bought Royal Scot Hotel in London in 2016 days after the referendum in which the U.K. voted to leave EU by a margin of 52% to 48%.

"I believe this deal on the table is the best obtainable deal to avoid a hard Brexit," he said. However more needed to be done to be competitive with the EU in the long term he added.

Singaporean property developer City Developments said the proposals did not change its view that the outlook for Central London's office market was positive.

"The short-term uncertainties surrounding Brexit have presented [City Developments] with off-market opportunities to acquire high performance assets with deep value," the company said.

Malaysia's Sime Darby Property said it remained committed to its 9 billion pounds ($11.7 billion) Battersea Power Plant upscale property development.

The group is one of the three parties in a Malaysian consortium that is turning the 170,000 sq. meters pre-World War Two industrial land in London into a retail and living space.

Arthur Tan, chief executive of AC Industrials, the manufacturing unit of Philippine conglomerate Ayala, which last year took over in a U.K. contract manufacturer, STI Enterprises, said, "Currently any settlement between the U.K. and the EU is positive to our business."

"Our current customers operate in both sides of the Atlantic and since [the group] has manufacturing and engineering options at both sides, we remain positive that we are able to manage either the continued status or a resolution," Tan said.

Meanwhile the U.K. may have to do more to entice rapidly growing Asian startups. For Jasen Wang, founder of Chinese robotic company Makeblock, the proposed Brexit agreement has failed to ease his worries over freedom of movement across borders with European countries.

The company, which has a sales office in Amsterdam and plans to expand its presence in Europe, but said that the U.K. will be left out of their consideration when it comes to deciding the location of their next branch.

"We are concerned about visa issues," Wang said. "If we have an office in an EU member state, our staff could travel around the region visa-free. But if our office is in the U.K., how easy would it be for them to visit and work in Europe?" he said.

Cliff Venzon, Kim Jaewon, Nikki Sun, CK Tan, Mitsuru Obe, Coco Liu, Mayuko Tani, and Akihide Anzai contributed to this report.