Around 20 UK personnel, including three pilots, "embedded" with coalition forces from the US and Canada were reportedly involved despite UK MPs voting in 2013 against military action in Syria.
Details of British personnel's involvement in strikes by allied nations' forces were disclosed by a freedom of information request from the pressure group Reprieve.
The UK's Ministry of Defence (MoD) said personnel embedded with the US, French and Canadian armed forces had been "authorised to deploy with their units to participate in coalition operations" against ISIS.
The MoD said UK pilots were not currently taking part in the region, but ministers would have been aware of their recent role.
Jennifer Gibson, a lawyer from Reprieve, said: "Documents obtained by Reprieve indicate that UK personnel have already been involved in bombing missions over Syria for some time - making the current debate over whether Britain should carry out such strikes somewhat obsolete."
The US and five Arab allies launched the first strikes against IS militants in Syria in September last year.
Last month UK defence secretary Michael Fallon urged British MPs to consider backing air strikes on ISIS in Syria and said the extremists needed to be targeted "at source".
Britain has already been carrying out surveillance and air-to-air refuelling operations over Syria.
Opposition Labour has indicated it would not oppose military action in Syria.
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