The Home Office is scrapping its target of processing most asylum claims within six months, the Guardian has learned.

Human rights lawyers expressed alarm at the news, saying the number of vulnerable asylum seekers facing delays in having their claims processed could become even worse than its current level.

One said the decision was likely to lead to an increase in legal challenges against the Home Office if applications are not processed promptly.

In 2014 the Home Office introduced a service standard to process 98% of straightforward asylum claims within six months.

The Guardian revealed last August that the Home Office has left some people waiting more than 20 years for decisions on their asylum claims. Seventeen people received decisions from the Home Office in 2017 on claims they had submitted more than 15 years ago, four of whom had waited more than 20 years for a decision. The worst case was a delay of 26 years and one month after the person initially applied.

A report from the charity Refugee Action showed that at the end of 2017, 14,306 people were waiting longer than the Home Office’s six-month target, a 25% increase on the previous year.

Of the decisions the Home Office made in 2017, 18,189 or 75% were taken within six months of application, 2,832 took between six months and a year, 3,059 between one and three years and 243 between three and five years.

Of the 40 people who waited more than five years to receive an answer, seven were granted asylum or another protection visa, 22 were refused and 11 either withdrew their application, left the country or died while awaiting a decision. They came from a range of countries, including Afghanistan, the Democratic Republic of the Congo, Eritrea, Somalia and Yemen.

Hannah Baynes, of Duncan Lewis solicitors, said: “We very regularly see asylum seekers who wait over six months for an initial decision. In many cases, we have no option but to resort to issuing judicial review proceedings to challenge the delay in a decision being made in a client’s asylum claim.

“I am concerned about the impact on clients’ health if the Home Office is planning to abandon its current target of six months for initial decisions in asylum claims. Such a practice creates uncertainty and means that those seeking asylum in the UK are unable to move on with their lives.

“I also consider that this will result in the courts’ time and public funds being used up when judicial review proceedings are issued to challenge the delay in decisions being made in clients’ asylum claims.”

A Home Office spokesperson said: “We are committed to ensuring that asylum claims are considered without unnecessary delay, to ensure that individuals who need protection are granted asylum as soon as possible and can start to integrate and rebuild their lives, including those granted at appeal.

“We have moved away from the six-month service standard to concentrate on cases with acute vulnerability and those in receipt of the greatest level of support, including unaccompanied asylum-seeking children (UASC).

“Additionally, we will prioritise cases where an individual has already received a decision but a reconsideration is required. We are engaging stakeholders to help inform how we will prioritise decision-making in the future, which will result in a new service standard that will seek to address the concerns that have been raised with the current arrangements.”