A Christian human rights activist is at the centre of a diplomatic storm between the UK and China after he was denied access to Hong Kong without explanation.

Benedict Rogers, a prominent activist and vocal critique of human-rights abuses in China and south-east Asia, was blocked from entry into the former British colony on Wednesday.

He told Christian Today he was not given any reason for being barred and was 'shocked' by the decision, saying it raised concerns about freedom of expression and even freedom of religion in the region.

The UK foreign secretary Boris Johnson weighed in, saying he 'very concerned' about Rogers' treatment and was seeking an 'urgent explanation' from the Hong Kong authorities and the Chinese government.

Johnson said: 'I am very concerned that a UK national has been denied entry to Hong Kong.

'The British government will be seeking an urgent explanation from the Hong Kong authorities and from the Chinese government.

'Hong Kong's high degree of autonomy, and its rights and freedoms, are central to its way of life and should be fully respected.'

The city was handed back to the Chinese government in 1997 under an agreement it would be run under a 'one country, two systems' arragement, meaning the city had its own legal system, some democracy with multiple political parties and human rights such as freedom of assembly and free speech.

But Rogers told Christian Today he was convinced Beijing was behind his treatment after being warned by a contact in the Chinese embassy in London there were concerns about his trip, despite insisting it was 'private and personal' and not on political business.

'Obviously they are very nervous of anyone who is critical of the erosion of freedom in Hong Kong,' he told Christian Today.

'My treatment speaks of that erosion of freedom.' He added the decision to deny a private British citizen without any criminal convictions was shocking.

'The only explanation is the Chinese,' he said. 'The decision was taken by the Chinese regime.'

The Chinese Foreign Ministry spokeswoman Hua Chunying hit back at the UK's criticism on Thursday saying the decision 'to allow or not to allow someone to enter is [within] China's sovereignty'.

She told a press conference: 'Whether this person's trip to Hong Kong involved an intention to intervene in Hong Kong's internal affairs and judicial independence – he knows very well himself.'

Rogers said their approach was 'counter-productive' but added it 'has actually shown a lot of the reality' of the level of freedom in the city.

'If China is now in control of Hong Kong immigration, it means that the idea of Hong Kong people running Hong Kong is dead, and if China decides to deny entry to Hong Kong to a person simply for wanting to meet people who have a range of political views, it means the basic rights of freedom of expression and freedom of association are undermined.'