Theresa May was accused of backing down on security concerns about Chinese involvement in nuclear power and failing to drive a better deal for taxpayers, following the announcement on Thursday on building the £18bn Hinkley Point C plant.

May’s surprise decision to review the first planned nuclear power station for a generation when she arrived in Downing Street had been regarded as marking a break with George Osborne’s enthusiastic courtship of China, and a greater willingness to take on big business. But after the Chinese ambassador publicly raised concerns about future trading relationships if Britain pulled the plug on the deal, May gave the go-ahead.

On Thursday Vince Cable, the former Liberal Democrat business secretary, said that the prime minister had ducked an opportunity to revisit the economics of the Hinkley contract – which will entail the Treasury promising a guaranteed floor price for the electricity generated, paid for through householders’ bills, over the reactor’s lifetime.

“Over the last few weeks we have been marched up the hill and back here,” Cable said. “Maybe she just wanted to demonstrate that this had her fingerprints on it.The biggest issues around Hinkley are the price, and the technology, and those remain unchanged. The big question is about the economics of it, and that is still unresolved, and worrying.”

The government has guaranteed a price of £92.50 to EDF for every megawatt hour of electricity generated, despite concerns that that is more than double the current market rate.

Cable called for May to take a tougher approach to other looming infrastructure decisions, including the proposed third runway at Heathrow, and the HS2 high-speed rail project. “There are problems with each of the three Hs,” he said.

The government insisted the Hinkley Point C nuclear plant in Somerset was being approved with “significant new safeguards” to make sure China and other foreign investors could not own stakes in British nuclear plants without UK government approval.

However, Labour and environmental groups described the new security measures as mere “window dressing” and “hot air”, which would change very little about the project, since the price would remain the same and the power to block mergers on security grounds already existed in law.

Under the new proposals, EDF, the French firm building the plant with a £6bn investment from a Chinese-state nuclear firm, will not be able to sell on its ownership without permission. For future projects, the UK government will own a “special share” that means it will have a veto over owners if there are national security concerns.

But in a signal that the requirements may not be very stringent, both EDF and China General Nuclear (CGN) said they were delighted by the approval, which they claimed would let them proceed with Hinkley and their wider plans for nuclear construction in the UK.

The shadow energy secretary, Barry Gardiner, said the government had “backed down with a whimper” over its security concerns about Chinese involvement, saying it had announced “new powers they already possess”.

He told Greg Clark, the business secretary: “They have claimed they introduced significant new safeguards into this package. The secretary of state already has such powers. Will the secretary of state acknowledge that he can prevent the sale of any element of critical infrastructure?”

John Sauven, executive director of Greenpeace UK, said the government’s review seemed “to have been a lot of hot air”.

The Chinese are particularly keen to proceed with a new nuclear plant at Bradwell in Essex because it will be their own design built under the UK’s tough safety regulations, allowing the company to use it as a showcase to the rest of the world. CGN sources indicated they hoped to submit their formal plans for approval this autumn.

“We are now able to move forward and deliver much-needed nuclear capacity at Hinkley Point, Sizewell and Bradwell with our strategic partners, EDF, and provide the UK with safe, reliable and sustainable low-carbon energy,” CGN said. “CGN and EDF have worked together in close cooperation for decades and this has laid a solid foundation for these three new nuclear projects. CGN looks forward to leveraging its 30 years’ experience in nuclear construction and operation and playing an important role in meeting the UK’s future energy needs.”

Clark said the new security restrictions were an improvement, claiming EDF would have otherwise been at liberty to sell its stake on. His department for business, energy and industrial strategy confirmed that this was the only new security requirement for Hinkley to proceed.

Before an announcement in the Commons, Clark said: “Britain needs to upgrade its supplies of energy, and we have always been clear that nuclear is an important part of ensuring our future low-carbon energy security.”

The French prime minister, Manuel Valls, tweeted that it was excellent news that the Hinkley project was going ahead and that it would be a boost for jobs in France. Michel Sapin, the French finance and economy minister, said it was an “undeniable success for French industry” and would be “a big contribution to supporting employment in France”.

Jean-Bernard Lévy, chief executive of EDF group, said: “The decision of the British government to approve the construction of Hinkley Point C marks the relaunch of nuclear in Europe.”