UK police were "complicit" in the radicalisation of three sisters thought to be in Syria with their nine children, lawyers for the families say.
Officers encouraged the Bradford women to contact their brother - believed to be fighting in Syria - with "reckless disregard" for the consequences, the lawyers told MP Keith Vaz in a letter.
One mother said she left the UK due to "oppressive" surveillance, they added.
West Yorkshire Police said it rejected the claims.
Assistant Chief Constable Russ Foster said: "We completely reject accusations that the police were complicit in the alleged grooming of the missing family or that we were oppressive to them."
Khadija, Sugra and Zohra Dawood and their children went missing on 9 June, and an Islamic State (IS) smuggler has since told the BBC they have reached Syria.
In the letter to Mr Vaz - chairman of the Commons Home Affairs Committee - solicitors representing two of the women's husbands, Akhtar Iqbal and Mohammed Shoaib, said they were "extremely disappointed" with the police's handling of the case.
Voicemail from Syria
The letter said officers from the North East Counter Terrorism Unit (NECTU) had been "actively promoting and encouraging" the three sisters to contact their brother, Ahmed Dawood.
He is believed to be fighting alongside extremists in the country, parts of which are controlled by IS militants.
"It would appear that there has been a reckless disregard as to the consequences of any such contact on the families," the solicitors said.
The letter continued: "Plainly, by the NECTU allowing this contact they have been complicit in the grooming and radicalising of the women."
The letter also stated that Zohra Dawood left a voicemail for her family on 17 June, confirming she was in Syria and saying she and her sisters travelled "due to the oppressive nature of the continued surveillance by the police".
The lawyers said police had refused to give their clients information about the case, and had been "anxious to ensure that there is no criticism of them and their tactics".
The home secretary and the foreign secretary will also be receiving letters complaining about the police, the lawyers added.
The Home Office said it had received the letter and would respond in "due course".
"Our priority is to dissuade people from travelling to areas of conflict and the Prevent strategy is working to identify and support individuals at risk of radicalisation," it said in a statement.
"We have introduced the Counter-Terrorism and Security Act to disrupt the ability of people to travel abroad to fight and then return, enhance our ability to monitor and control the actions of those who pose a threat, and combat the underlying ideology that feeds, supports and sanctions terrorism."
Labour MP Mr Vaz told the Mail on Sunday the lawyers' claims were "concerning".
"That three women could disappear from the UK to take nine children into the heart of a war zone is incomprehensible," he added.
Mr Vaz has issued a statement saying he will meet the two fathers on Monday.
BBC correspondent Danny Savage said the letter "raises questions that the police now have to answer".
The sisters and their children went missing following a religious pilgrimage to Saudi Arabia.
They travelled to the Saudi city of Medina on 28 May and were last seen in a hotel there.
They were expected to fly back to Manchester, but their husbands reported them missing when they did not return.
On Tuesday, Mr Iqbal and Mr Shoaib said they "could not live" without their families and begged them to return.
NECTU said it was "continuing to make extensive enquiries" in order to try to bring the women and children home.
Dr Afshin Shahi, of the University of Bradford, told the BBC his research suggested people who "already have contacts in transnational jihadi movements" were more likely to be "encouraged" to join IS.
But he said he did not think the women's brother in Syria was the "only factor" in their decision to go there.
He added that it was "very naïve" to suggest police had played a "leading role" in the decision.
"There are multiple factors - the issue of identity, the issue of religion, ideology, foreign policy, personal circumstances… that can actually influence the process," he said.
"We have to remember that radicalisation is not an event; radicalisation is a process. And for that reason I really encourage observers to refrain from providing a simple answer for a difficult question."
In February, emails were published in which Mohammed Emwazi - the militant known as "Jihadi John" - claimed he had been harassed by police before he left to join IS.