British military pilots embedded with coalition allies' armed forces have taken part in airstrikes against the Islamic State group in Syria, despite the country's lawmakers voting against allowing U.K. armed forces to take part in military raids over Syria.

The revelations came in response to a freedom of information request from human rights advocacy group Reprieve. Britain's Ministry of Defense revealed that 20 U.K. military personnel, believed to include three pilots, were embedded with U.S. and Canadian forces who were conducting operations against ISIS in Syria, the BBC reported. The troops are on an exchange program, and are effectively operating as foreign forces, officials added.

The U.K. parliament voted in 2013 to authorize air strikes against ISIS in Iraq, but not in Syria. However, Prime Minister David Cameron raised the possibility late last month that British forces could begin targeting militants in Syria, after a terrorist attack on a beach resort in Tunisia killed 38 people, 30 of whom were British. Cameron is expected to call a vote in the parliament on the issue later this year.

Britain has been providing limited support to coalition military action in Syria, carrying out refueling and surveillance operations over the country, the Telegraph reported.

British lawmakers who had voted against military action in Syria reacted angrily to the disclosure. Conservative MP John Baron called for an immediate end to U.K. military strikes in Syria and urged Defense Secretary Michael Fallon to explain himself to parliament.

“What this does show is at the very minimum an insensitivity to parliament’s will,” he said, according to a Guardian report.

“We voted in 2013... that there should be no British military intervention in Syria. We were told that No 10 [Downing Street, the Prime Minister's official residence] had got the message and that any future intervention would be subject to a vote,” he added.

The U.S. and other countries in the coalition have been carrying out airstrikes against ISIS in Syria since September 2014.

A spokesperson for Reprieve told BBC that the revelations now made obsolete any public debate about whether to engage in military action in Syria, and demanded the government “come clean” about what it was already doing.