Lam said they presented the latest information and legal arguments to staff director Katie Gunn, including advice from a British senior counsel that there were sufficient grounds to launch an investigation under the country’s jurisdiction.

Leung fired back with his statement, posted on Facebook on Saturday morning, or midnight London time.

Now a vice-chairman of the Chinese People’s Political Consultative Conference, China’s top political advisory body, Leung said he had received a letter from the NCA – without specifying when.

He posted only an excerpt: “The NCA has now completed a comprehensive review of the material provided to date.”

The letter further stated: “The NCA has also conducted its own inquiries and made every reasonable effort to obtain corroborating material in order to reach a decision ... After careful consideration, the NCA has concluded that on the basis of the information acquired, there is insufficient evidence to progress the investigation.”

Leung then wrote: “Lam Cheuk-ting reported the case to the NCA in a high-profile way a few months ago and kept chasing the matter. There is no way he did not know that the NCA had made a decision.”

Lam confirmed that he received the email, only when he woke up in London on Saturday. He disclosed the entire email dated on Friday from NCA’s Prosperity Director Donald Toon, who wrote him the same paragraphs as Leung posted.

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Toon declined Lam’s request to meet representatives of the agency and said this would not be possible as doing so was “unlikely to alter our assessment”.

In a statement, Lam and Wan clarified they were not told about the progress of the investigation during their meeting with Gunn on Friday, but were asked to email them supplemental information.

“We are very disappointed by the NCA’s decision,” they wrote. “It is difficult to understand why the NCA is no longer following up the case, and we will demand the NCA explain its rationale.”

The pair said they would liaise with their legal teams in both Hong Kong and Britain for their next move, besides calling on British lawmakers to pursue the case and studying plans to follow it up in Australia.

The Democratic Party has raised more than HK$2,000,000 from more than 6,400 donors after it launched a crowdfunding campaign to bankroll an international investigation into Leung’s case.

Wan said the fund was operated on a reimbursement basis and the surplus, if any, would be donated to charity. According to the last report disclosed in June, around HK$480,000 had been spent.

“We understand both the expectations of the public and the difficulties of following up Leung Chun-ying’s UGL case through civil society,” Lam and Wan said. “No matter how difficult it is … we must go all out to thoroughly investigate and not let the public down.”

Leung struck the lucrative deal following UGL’s 2011 purchase of DTZ, a property services company once listed in Britain, of which he was a director. As part of the bargain, he agreed not to form or join a rival firm and to help promote the company.

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A Legco delegation is visiting parliaments in London and Edinburgh from now until next Sunday.

Lam and Wan arrived early on Thursday and met Labour Party politician Catherine West and Transparency International, a global anti-corruption NGO, in their bid to advance the investigation into Leung.