A Foreign Office minister has been criticised for failing to condemn a law that bans political criticism on the internet in Vietnam. The law came into force 24 hours before the minister, Mark Field, arrived in the country.

Field, the Asia and Pacific minister, arrived in Vietnam Wednesday morning and sent out a tweet saying that media freedom would “help Vietnam realise its enormous potential”. He also said in an article in a Vietnamese newspaper that Britain would strengthen its relationship with the one-party communist state after Brexit.

On 1 January Vietnam introduced a cybersecurity law forcing internet providers to censor content deemed “toxic” by officials.

Field said the UK would strengthen its cooperation over Vietnamese cybersecurity. Nearly £5m in telecoms intercept equipment has been approved for sale to Vietnam since 2015. Campaigners against the arms industry say that action could be used to implement the crackdown on free speech.

A free press and a free internet help economies grow and innovate; media freedom will help Vietnam realise its enormous potential. Fascinating discussion with those involved in the sector here @UKinVietnam @GarethWardUK pic.twitter.com/5R778JcGxe — Mark Field MP (@MarkFieldUK) January 2, 2019

Responding to Field’s comments, Phil Robertson, the deputy Asia director for Human Rights Watch, said there was no substantive ground to believe Vietnam was prepared to end its crackdown on freedom of speech and civil society.

Robertson said: “The reality is, freedom of the press doesn’t exist in Vietnam because the government exerts effective control over all media outlets in the country. Up until now social media has been the outlet for free expression but the Vietnam government now has a draconian new law on cybersecurity which it can use as the hammer it needs to shut down online discussions and prosecute persons for what they say.

“The UK should publicly demand the revocation of the cybersecurity law and bend over backwards to ensure that no British government programmes or foreign investment do anything to facilitate the deepening crackdown.”

The Foreign Office said that Field raised the issue of media freedom including specific concerns about the new law with his Vietnamese counterparts on Wednesday, he also convened media freedom experts and activists to discuss the issue in Hanoi on the same day. They said the tweet was issued to accompany that event and said that the tweet said that Vietnam must have media freedoms in order to realise its enormous potential.

Despite the human rights situation in Vietnam, UK officials have cultivated the country as a customer for British military and security technology, approving £77m of sales of weapons and dual-use technologies since the Conservatives gained power in May 2015.

Vietnam was one of a number of countries invited to shop for surveillance equipment at a Home Office-sponsored arms fair in March 2017; months later civil servants from the Defence and Security Organisation, the government body that promotes arms manufacturers to overseas buyers, joined 14 different UK arms companies on a promotional tour of the country.

In November last year Vietnam’s deputy minister of national defence, Nguyen Chi Vinh, visited London for a joint “defence policy dialogue” summit co-chaired by Earl Howe, the minister of state for defence.

Among the most concerning exports, according to Andrew Smith, a spokesman for Campaign Against Arms Trade, was £5m worth of telecommunications interception equipment. “Interestingly one licence was refused in 2017, so the [UK] government clearly thinks there is some degree of risk, but has allowed 18 other licences since May 2015.” The most recent was in January 2018, Smith said, adding: “Surveillance equipment is particularly sensitive when put in the hands of authoritarian states.”

A Foreign Office spokesperson also said that media freedom was a priority for Jeremy Hunt, the foreign secretary, which was why he was hosting an international conference on the issue in London this year.