Plans to curtail the number of jury trials in England and Wales have been described as “unpopular, untested and poorly evidenced” by thousands of lawyers who have written to the prime minister.

The letter to Keir Starmer, a former director of public prosecutions, from 3,200 lawyers, including 300 senior barristers, comes as his government faces the prospect of one of its most serious backbench revolts since coming to power.

Efforts by David Lammy, the justice secretary, to change the mind of one of the leading Labour figures opposed to the plans, the backbencher Karl Turner, failed after the men met last night.

Turner, who had previously co-ordinated a letter from 38 Labour MPs urging the prime minister to reverse the plans, said he had “absolutely not” been convinced.

The Conservatives are expected to force a vote to try to block the second reading in parliament on Tuesday. However, the true scale of the Labour rebellion may not yet be evident.

More than 65 Labour MPs are thought to be considering voting against it, but it is understood that many may abstain and instead vote against it at a later stage of the legislative process, such as report stage.

On Monday Lammy warned opponents of the bill that criminals would walk free if it was blocked.

“Across Britain today, too many victims endure the same ordeal. For them, justice delayed becomes justice denied. When that happens, offenders are left free to roam our streets, and more victims are created,” the deputy prime minister wrote in an opinion piece for the Telegraph.

Lammy described the scale of the courts backlog, which the bill is aimed at addressing, as stark. He said the number of cases waiting to be heard in crown court had almost doubled, from about 38,000 in 2019 to nearly 80,000 today.

Lammy appealed to Labour MPs’ sense of social justice as he made the case for the plans last night at a meeting of the parliamentary Labour party, telling them: “When a public service collapses, it is never the wealthy or the well-connected who fall through the cracks first.”

The letter to Starmer came from thousands of legal professionals, including a former director of public prosecutions, Sir David Calvert-Smith, and was organised by the Bar Council.

It said: “We write in our capacity as legal professionals with extensive experience of working in courts across England and Wales at all levels and in all jurisdictions, to request that you rethink proposals to remove jury trials for cases with anticipated sentences of up to three years’ imprisonment.

“We fully support and share the government’s aim of bringing down the backlog in the criminal courts and reducing delays between alleged offence and completion of the case. We have long warned that the criminal justice system is in crisis.

It added that juries had not “caused this crisis” and cited the findings of the independent review of the criminal courts (the Leveson review), in which Sir Brian Leveson stated that “the most significant cause is chronic underfunding at every step”.

The government’s overhaul of the justice system would also involve the Ministry of Justice ramping up use of AI in courts to cut backlogs, and rape victims getting free legal advice throughout the criminal justice process, as part of plans to make the experience less traumatic.