British Prime Minister Theresa May on Sunday defended her decision to delay a partly Beijing-backed nuclear power project, despite it causing diplomatic tensions, as she arrived in China for a G20 summit.

Beijing has a one-third stake in the plan to build Britain's first nuclear plant in decades at Hinkley Point in southwest England, along with French company EDF.

May unexpectedly delayed the project in July after EDF gave it a green light, but gave no clear reason.

"The way I work is that I don't just take an instant decision. I actually look at the evidence, take the advice, consider it properly, and then come to a decision," May said at a press conference.

"I've been very clear that I will be doing that and will be taking a decision sometime this month," she added.

May has reportedly suspended the project on national security grounds, echoing fears which caused Australia to block a major electricity grid deal with a Chinese consortium last month.

Critics cite the enormous cost of the PND 18-billion (21-billion-euro, $23-billion) project as well as security concerns about the involvement of China's major energy group CGN.

China's ambassador to Britain last month urged London to approve the plant as soon as possible, warning that relations between the two countries were at a critical point, as state media said China "cannot tolerate" accusations that its involvement threatened British security.

The nuclear deal had been seen as a cornerstone of a "golden era" of Chinese-British ties pushed by then-prime minister David Cameron and finance minister George Osborne.

British media said the move was a sign that May took a more sceptical view on ties with China than her predecessor.

The project has been criticised for committing British taxpayers to paying above-market rates for electricity for decades to come.