There are calls for Priti Patel to withdraw or justify claims she made before parliament that most people who travel to the UK in small boats are not genuine asylum seekers.

Two Labour peers, David Blunkett and Shami Chakrabarti, have also questioned whether the home secretary has evidence that backs her claim that “70% of individuals on small boats are single men who are effectively economic migrants”.

There is deepening concern from refugee charities that the government is feeding an untrue narrative that claims migrants who travel to the UK by boat are undeserving of sympathy.

At the Lords home affairs and justice committee last week, Patel was questioned about her new policy of deeming any person who arrives in the UK to claim asylum after passing through a “safe” country as “inadmissible” – meaning their claim will not be considered.

Describing the people who would fall into this category, she said: “In the last year, 70% of individuals on small boats are single men who are effectively economic migrants. They are not genuine asylum seekers.”

Home Office officials were asked to provide data that backed up the home secretary’s assertions.

The Home Office has declined to give an official response to a request from the Guardian. A source responded by saying that of 8,500 people arriving by small boat in 2020 87% were men and 74% were aged between 18 and 39 – but did not provide evidence related to their asylum claims.

Lady Chakrabarti, the former head of Liberty and a member of the committee, said Patel’s comments should be properly explained, corrected or withdrawn.

“Both the refugee convention and the lives of desperate people are too precious for the home secretary’s 70% statement to go unchallenged,” she said.

“Is the Home Office really saying that most single male asylum seekers or most people crossing the Channel are ‘economic migrants’ and unworthy of refuge? If yes, what happened to giving anxious scrutiny to every individual claim for asylum?”

Lord Blunkett, the former Labour home secretary who asked Patel the question that provoked the “70%” response, said the government could struggle to justify the figure because officials were still processing a backlog of asylum claims.

“It is not surprising that the Home Office has not responded [to the Guardian’s request to justify the 70% claim] given that the backlog of cases is so extreme. The latest figure I saw was 125,000, which exceeds many previous years.

“There is a presumption being made by the home secretary about their claims before they have had an opportunity to make their claims. Until the backlog is massively reduced we won’t know about those arrivals who come over the summer,” he said.

In oral evidence to the home affairs select committee in September 2020, Abi Tierney, the director general of UK Visas and Immigration, stated that of 5,000 people who had crossed in 2020 until the date of the committee meeting, 98% had claimed asylum.

Published Home Office data shows that many of those arriving on small boats whose claims were deemed inadmissible originate from conflict zones such as Iran, Iraq, Afghanistan and Sudan.

Dr Peter Walsh, a researcher at the Migration Observatory at the University of Oxford, said: “A majority of asylum claims (including those of people who arrived in the UK by small boat) are ultimately successful. Specifically, the Home Office reports that 59% of claims filed in 2017 to 2019 inclusive were ultimately successful, accounting for appeals.

“It is reasonable to suppose that Channel migrants will be more likely than other asylum seekers to have their asylum claims granted. This is because the nationalities that are most common among Channel migrants – like Iranian, Syrian, Afghan and Yemeni – have a higher-than-average likelihood of ultimately being successful. It is not clear what evidence exists to support the claim that 70% of Channel migrants are economic migrants. But on the basis of the available data, it seems unlikely that no more than 30% of Channel migrants would have their asylum claims accepted.”