24/09/2015 11:31 BST | Updated 24/09/2016 06:12 BST

An Effective Counter Terrorism Strategy Must Uphold the Sanctity of the Law

Today, in conjunction with the Green peer Jenny Jones and international human rights group Reprieve, I launched a legal challenge to force the UK Government to come clean over its 'targeted killing' of people in countries where Britain is not at war. It's a response to what the Prime Minister himself has described as a "new departure" for the country, referring to the drone attacks of 21 August during which two British citizens were killed in Syria. The Prime Minister justified the attacks by claiming the men were terrorists suspected of plotting against the UK.

Like him, I want to stop ISIS in its tracks. I deplore the use of violence and think the UK should be acting decisively to counter terrorism. I am not involved in this legal challenge because I want to defend terrorists, as some are claiming. I am doing it because I want to defeat them: and think that can be done most effectively in the long term if the Government acts in accordance with domestic and international law.

And yet at the moment we cannot tell if this is the case. The Government has refused to publish the legal basis for action, creating a legal and accountability vacuum. Without our legal challenge it's impossible to determine whether the decision to execute to British men - and what that signifies in terms of Government policy - meets the robust conditions set out in international and domestic law.

Those conditions include the right to a fair trial when charged with a crime. The Prime Minister has outlined why that's not practical in relation to the Raqqa drone strike, including because there's nobody with whom to negotiate even an arrest. But that doesn't just clear the way for state sanctioned murder of British citizens. We need to know what imminence means when the Prime Minister talks about these men planning an imminent attack. And we need the answers to all sorts of other questions too, including whether the legal advice the Prime Minister received applies just to this case or will be used to target British citizens in future; whether every possible avenue was explored in terms of capturing, detaining and trying the men; and whether this confirms that the UK government's official position is not to afford its citizens abroad any of the protections afforded by the Human Rights Act.

All of these questions underpin the development of a strong and effective counter terrorism strategy.

It's also imperative that such a strategy is developed in proper consultation with Parliament - albeit, of course, that individual attacks can rarely be decided or discussed in this forum - and that MPs are given the information and opportunities to hold the Government to account for its actions. A statement from the Prime Minister does not constitute accountability.

Moreover, adopting a 'Kill Policy' in secret, as the Government appears to have done, means there's no prospect of proper independent scrutiny. If a police officer needs to use fatal force against a terrorist on the streets of London, there's policy guiding what they can and cannot do and the IPCC would automatically have oversight. Yet when a drone is used to kill a suspected terrorist, there's no similar framework. Sanctioning lethal drone attacks on British citizens is a significant departure from previous policy, as well as potentially unlawful, and it's deeply concerning that it has occurred without appropriate oversight.

The importance of this is underlined by the apparent contradictions in the way that the Government has presented its justification for drone strikes in Syria. On the one hand, the Prime Minister claimed that the strikes were an act of self-defence against an imminent attack on Britain. On the other, in a letter to the UN Security Council, the Government additionally cited the collective self-defence of Iraq. I am concerned that this latter point may have deliberately been excluded from the Prime Minister's statement to MPs because it represents a complete violation of Parliament's 2013 vote against any form of military intervention in Syria.

There's also the crucial question of whether drone attacks like those carried out by the UK are counterproductive when it comes to combatting terrorism. In the US, whose drone programme has killed hundreds of civilians in countries such as Pakistan and Yemen, thanks to faulty intelligence and a lack of safeguards, former heads of defence intelligence and CIA station chiefs have called the drones a 'failed strategy', that can end up 'creating more enemies than you are removing from the battlefield'.

By invoking the right to self-defence, the UK has treated terrorists not as criminals but as soldiers, according them warrior status and so playing into ISIS' hands: I fear the Government has walked through a door held open by the terrorists and there are policy implications for how we defend ourselves in future.

It may well be that the legal challenge launched today confirms the UK Government acted lawfully in killing these British citizens. That will give us some welcome clarity. The case should help Parliament hold the Government to account in future, because MPs will know more about what the policy on targeted killings actually is. And let's remember the Prime Minister made it clear in his Statement this may not be an isolated incident. And my hope is that by bringing this case we can also start an evidence based debate about how best to defeat the terrorists.

ISIS and their ilk are barbaric. Ultimately we best challenge their hateful ideology and actions by protecting the sanctity of life and the law.

Caroline Lucas is the Green Party MP for Brighton Pavilion