Sir Keir Starmer, the shadow Brexit secretary, will fly out to Taiwan this weekend to lobby against the death penalty in an initiative also aimed at demonstrating the UK’s continued commitment to international legal standards following the EU referendum.

His four-day trip, which is supported by the Foreign Office, will include meetings with the country’s vice-president, minister of justice and senior judiciary. There are more than 40 prisoners on death row in the republic.

Starmer, who was formerly director of public prosecutions, has for many years been a director of the Death Penalty Project, which campaigns for abolition of capital punishment across the world.

According to Amnesty International’s latest estimates, 142 countries have stopped imposing capital punishment in law and practice out of more than 190 states globally. Asia remains one region where the death penalty is still widely used.

Taiwan adopted a moratorium on using the death penalty but on 31 August, Lee Hung-chi, who was convicted of murdering his ex-wife and daughter, was shot through the heart from behind – the country’s routine method for carrying out executions.

Starmer, who visited Taiwan two years ago, told the Guardian: “The most important thing is to ensure that the moratorium stays in place. One of the most important points to emphasise after the referendum is the UK’s continued commitment to international law … across the world.

“For me personally, and the country, compliance with international human rights obligations has always been central and I have devoted much of my career to that cause ... Going to Taiwan is part of maintaining the UK’s commitment to international human rights.”

Saul Lehrfreund, a solicitor at the London law firm Simons Muirhead & Burton and co-founder of the Death Penalty Project, is accompanying Starmer to Taiwan. He said: “We will be raising the case of Lee. Taiwan has not said it will abolish the death penalty tomorrow, but has stated that it’s a long-term objective.”

Lehrfreund has also been to talk to authorities in China about the death penalty. Chinese executions have declined in number: 25 years ago as many 25,000 prisoners were being killed every year. The current figure is believed to be roughly 3,000 executions.

“The reality is that miscarriages of justice are inevitable anywhere in the world. An individual on death row in Taiwan was exonerated recently after spending 10 years on death row.”

The UK’s commitment to opposing the death penalty has been questioned since the government signalled in July that it may allow two terror suspects, El Shafee Elsheikh and Alexanda Kotey, who have been detained in Iraq, to be extradited to the US, where they could face the death penalty.

The Foreign Office said: “It is a longstanding policy of the UK government to oppose the death penalty, in all circumstances, as a matter of principle.”

The department said it worked closely, through the Magna Carta Fund, with abolitionist organisations such as the Death Penalty Project to reduce the number of executions and restrict use of the death penalty.

Asked about the Isis suspects, the spokesperson said: “We are clear that any foreign fighters who may be captured in an armed conflict should be treated in accordance with international humanitarian law and brought to justice in accordance with legal due process.”

Starmer is due to return to the UK on Thursday after the end of the Conservative party conference. In an article he and Lehfreund have written for Taiwanese papers, they said: “The global experience shows that support for capital punishment dwindles after abolition as the punishment comes to be outdated.

“There are many unanswered questions about Lee’s death that make it sit so uncomfortably with a commitment towards abolition, including that his original sentence of life imprisonment was increased to death on appeal, the lingering and serious concerns as to Lee’s mental health and his apparent unwillingness to appeal or seek clemency … It has been made repeatedly clear that the death penalty is not an effective deterrent to crime.”