Asked how many people the government anticipated would come to the United Kingdom from Hong Kong, he said: "It's hard to give a precise forecast but I think it is fair to assume that only a proportion of those that are offered the new status that we set out in the House of Commons today will take it up".
But critics say the new law - passed by Beijing's rubber-stamp parliament this week without its text being released to the public - breaches the "One Country, Two Systems" principle that formally entered worldwide law in 1984.
In Washington, DC, the US House of Representatives joined the country's Senate in approving a bill that imposes sanctions on police units that have cracked down on Hong Kong protesters, as well as Chinese Communist Party officials responsible for imposing the new security law.More news: Chinese spyware has targeted Uighurs for years
Hong Kong leader Carrie Lam backed China new law calling it "most important development" since the city's return to Beijing's rule.
During Prime Minister's Questions, Mr Johnson told the Commons: "The enactment and imposition of this national security law constitutes a clear and serious breach of the Sino-British Joint Declaration".
China's Parliament adopted the law in response to protests past year triggered by fears that Beijing was stifling the city's freedoms, guaranteed by a "one country, two systems" formula agreed when it returned to Chinese rule.
"It violates Hong Kong's high degree of autonomy and is in direct conflict with Hong Kong Basic Law".
Labour said the government must "not waver" from its obligations to the people of Hong Kong and it expected the foreign secretary to lay out "concrete steps" to uphold the freedoms of those living there.More news: New Zealand's health minister Clark resigns
Britain and some two dozen Western countries urged China to reconsider the law, saying Beijing must preserve the right to assembly and free press.
"We firmly oppose this and reserve the right to take corresponding measures", it said without elaborating.
"Obviously, China is a leading member of the global community".
However, the ambassador appeared unrepentant following his visit, tweeting: "After turbulent months in the latter half of previous year, the national security law will bring the order＆stability to Hong Kong and get its economy back on track". Authorities have repeatedly said the legislation is aimed at a few "troublemakers" and will not affect rights and freedoms, nor investor interests.
This came on a day which saw hundreds of pro-democracy protesters arrested in Hong Kong as police enforced a new security law, on the 23rd anniversary of the former British colony's handover to China.More news: Hafeez, Shadab among six cricketers ready to join Pakistan squad in England
A police spokesman told Reuters the arrested man was surnamed Wong but could not confirm if he was leaving Hong Kong or working at the airport.