More than 40 international volunteers – a third of them British – who fought in Syria against the Islamic State terror group have written to the home secretary, Sajid Javid, to condemn his plans to prosecute UK citizens who remain in the country.

Four British families whose sons or daughters were killed fighting Isis have also signed the letter, raising concerns that Javid is “criminalising” those who risked their lives supporting the US-led coalition which two months ago defeated the IS caliphate.

They include the family of Anna Campbell, from Lewes, East Sussex, who was volunteering with the US-backed Kurdish Women’s Protection Units (YPJ) – the all-female affiliate army of the People’s Protection Units (YPG) – in Syria when she was killed by a Turkish missile last year. The 26-year-old was one of seven Britons killed fighting alongside Kurdish forces against Isis.

Last week Javid told British citizens in northern Syria to leave within 28 days or face a 10-year prison sentence if they attempt to return to the UK. Unveiling powers ostensibly drawn up to tackle jihadist terrorism, he is now accused of failing to distinguish between Britons in the jihadist enclave of Idlib, in Syria’s north-west, and those in the north-east, a region controlled by Kurdish forces that have been in the vanguard of efforts to defeat Isis.

Critics believe that Javid has not grasped the realities of the eight-year conflict, with British citizens able to legally visit parts of Syria controlled by the Syrian regime, repeatedly accused of war crimes and chemical attacks.

The most heated opposition to the home secretary’s Counter-Terrorism and Border Security Act has come from inside the autonomous Kurdish region in north-east Syria, known by the Kurds as Rojava.

“Using a law supposedly created to defend against Isis, you are criminalising as ‘terrorists’ those who’ve given more than any other British citizens in this struggle. Just two months ago, the whole world celebrated Isis’s defeat as a military force. That victory was led by the women of YPJ, along with the other Kurds, Arabs and Christians of north-east Syria – the democratic, women-led, autonomous region more commonly known as Rojava,” states the letter.

Hearing the news in March that the caliphate had been dismantled, Theresa May tweeted that the military victory “wouldn’t have been possible without the immense courage of … our allies”.

Signed by Dirk and Sofia Campbell, the father and sister of Anna, who was killed by Turkish air forces in Afrin, north Syria, in March 2018, the letter described her as one of the “British heroes” to have died defeating Isis.

It adds: “You have never honoured the bravery and sacrifice of volunteers in Rojava – you even refused to help bring back Anna Campbell’s body from Afrin.”

A Kurdish YPJ unit in Aleppo, Syria, in February 2018. Photograph: Omar Sanadiki/Reuters

The letter is also signed by Vasiliki and Chris Scurfield, parents of former Royal Marine Kosta Scurfield, who died fighting Isis with the YPG, which had billed itself as a 21st-century International Brigade, which fought fascism in 1930s Spain.

Others include Jane Lyndon, mother of Ollie Hall, a British 24-year-old who joined the YPG to fight Isis and was killed in Raqqa, while trying to save a Syrian child from a booby-trapped house. At his inquest the coroner paid tribute to his “outstanding example of courage and self-sacrifice”.

A YPG sniper, Jac Holmes, 24, from Bournemouth, was also killed in Raqqa trying to clear bombs planted by Isis. His mother, Angie Blannin, is a signatory.

The Kurdish militias were often described as the most dependable ground forces in the US-led coalition fighting Isis, losing more than 10,000 fighters as they pushed the jihadists from north-east Syria.

“You [Javid] have decided that simply travelling to or remaining in Rojava should be considered a terrorist act – even though the UK is part of the International Coalition against Isis, which relied on Rojava throughout its campaign,” added the letter.

Other signatories include 15 British volunteers with the Kurds, including Macer Gifford, 32, who fought Isis with the YPG, and Steve Kerr, a former British soldier from Northampton. They also include six US former and serving volunteers, two French, three German and seven Italians.

In total there are more than 50 signatories, including the radical intellectual Noam Chomsky, criminal defence lawyer Raj Chada, and a number of senior academics.