The discovery of 39 people dead in the back of a lorry in Essex has renewed focus on the risks taken by undocumented migrants to travel to the UK to seek safety and shelter.

A lack of safe and legal routes into the UK is in part driving a dependence on life-threatening methods including squeezing into the back of refrigerated lorries or riding in vulnerable dinghies across wild seas.

People fleeing the threat of torture, rape or death cannot claim asylum in the UK without physically reaching Britain, aside from a few limited programmes including the Syrian refugee resettlement programme. Family reunion routes – that is, those granted refugee status in the UK being able to apply to bring relatives to join them – have been drastically curtailed.

The UK gave protection – grants of asylum, humanitarian protection or alternative forms of leave and resettlement – to 18,519 people in the year ending June 2019, according to the most recently available figures.

However, it is relatively hard to secure asylum in the UK, which hosts less than 1% of the world’s refugees.

Furthermore, the hostile environment policy, including the UK’s use of immigration detention (it is one of the only countries in the world where the period of detention is not capped), is likely to have deterred some people from wanting to seek asylum at all for fear they will end up in purgatory.

The Dublin regulation, an EU law which states that an individual is eligible to be given asylum only in the first safe country they arrive in, encouraged people with good reasons for wanting to travel to the UK – for example, good command of the language – to attempt to do so by dangerous backdoor routes, often facilitated by ruthless people smugglers.

The Joint Council for the Welfare of Immigrants said: “Nobody should be in any doubt that the ultimate responsibility for these deaths lies with government policy, which has deliberately shut down safe and legal routes to the UK.

“We need more than empty expressions of sadness and shock from Priti Patel and Boris Johnson. We need a commitment to opening safe and legal routes to the UK and quick decisions for people seeking to make a better life for themselves in the UK. People move – they always have and they always will. Nobody should have to risk their life to do so.”

The National Crime Agency has responsibility for tackling what it calls organised immigration crime. This stands apart from modern slavery and human trafficking because it involves consent on the part of the individual travelling, while the latter involves coerced movement across borders. However, there is an obvious overlap and the incident in Essex could be either of the two.

A range of methods are used in organised immigration crime, including clandestine entry from near Europe, air-facilitated migration, use of false or fraudulently obtained documents and abuse of legitimate entry and leave to remain.

In 2018 new migrant camps emerged in the Grande-Synthe and Puythouck Lake areas near Dunkirk, where organised crime gangs are known to be active, recruiting migrants for onward movement to the UK.

Juxtaposed border controls remain the main targets for clandestine entry into the UK. These controls are part of an arrangement between the UK, France and Belgium whereby immigration checks on certain cross-Channel routes take place before boarding a train or ferry, rather than on arrival.

In 2018 the number of Eritreans detected at juxtaposed controls increased notably, with Eritrean nationals found concealed in refrigerated lorries and containers, probably indicating growing levels of organisation.

The Guardian understands that last year more than 35,000 attempts to cross the Channel illegally were prevented at the UK’s juxtaposed controls in northern France and Belgium. During the same 12-month period, more than 8,000 people were detected in the UK, either at ports of entry or having entered the country illegally on a vehicle.

The top 10 nationalities of undocumented people found at juxtaposed controls in 2018 were Eritrean, Iraqi, Afghan, Iranian, Albanian, Sudanese, Vietnamese, Pakistani, Syrian and Ethiopian.

What could be done in the UK to reduce the chance of individuals attempting such risky journeys? Refugee and asylum seeker rights campaigners have been calling for an extension to refugee family reunion rules to include a wider definition of family, allowing individuals safely based in the UK to bring family members to join them.

There have also been calls for the resettlement programme for Syrian refugees, which is run by the UN refugee agency, to be extended to other countries.

• This article was amended on 24 October 2019 because an earlier version referred to the Syrian refugee settlement programme as the only one which operates without the person reaching Britain. The Syrian programme is one of a small number of limited refugee resettlement schemes.