A decision as a backbencher to vote against one’s party ought not to be taken lightly. Political parties depend for existence and success, not so much on the holding of identical views, as on a shared philosophy and ties of loyalty and respect between members. So there are good reasons to try to find compromises when differences emerge on a specific matter. Last week, however, I voted against my party on a national issue for the first time in my 20 years in parliament. I felt I had no option, as the attempts I had made to get the matter resolved by compromise had failed. I also considered that the matter was far too important to be ignored.

The vote was not about stopping Brexit. As a matter of international law, Brexit will occur on 29 March 2019, and a refusal by parliament to approve a final deal cannot prevent this, as it could only happen by the agreement of the UK and each of the other 27 member states, either to rescind the article 50 notice or to vary the date of departure that article 50 prescribes.

The problem on which I focused is that, in order to take us out of the EU, the government is seeking, in clause 9 of the withdrawal bill, very extensive powers to implement whatever changes the withdrawal agreement it is trying to negotiate may require. Yet until we know what that agreement is, the need for these powers to rewrite laws by statutory instrument is hypothetical. The government has accepted that a completely separate piece of primary legislation will have to be enacted to approve any withdrawal agreement. But the powers being sought could be used to bypass the promised parliamentary scrutiny of the final agreement, or avoid its outcome. I and a number of Conservative colleagues considered this unacceptable. I was struck by the fact that at least one government minister questioned privately if clause 9 needed to be in the bill at all. I therefore sought an amendment to ensure the powers in clause 9 could only be used once the later legislation was passed by parliament.

Quick Guide What is the EU withdrawal bill? Show What is the EU withdrawal bill? The EU Withdrawal Bill – once known as the Great Repeal Bill – is going through the House of Commons to repeal the 1972 European Communities Act and transpose all existing EU legislation into domestic UK law, which will avoid a 'cliff-edge' change on the day after we leave the EU. Parts of the bill have been highly controversial, and MPs have tabled hundreds of amendments to try and change its wording, including a significant number of Conservative rebels. Some of the key controversies include its use of so-called Henry VIII powers, which will give government ministers the power to tweak the wording of laws to make sure they make sense in UK legislation - but those changes could take place without having to go through parliament. MPs have called this a "power grab" by the government. The government estimates around 800 to 1,000 measures called statutory instruments will be required to make sure the bill is applied correctly. Other concerns include the government's decision not to include the EU charter of fundamental rights in the law being transposed. Other amendments are attempts to affect the Brexit process, including legislating for a transitional period and giving MPs a binding meaningful vote on the deal secured by Theresa May, before the deal is finalised.

Unfortunately the process of negotiation that seems to have been working reasonably well on other issues in the bill broke down. I cannot be certain why this was the case, but it is hard to avoid the conclusion that a decision was taken to deliberately face us down. We were offered assurances that did not adequately address the problem we identified , and that is why we put the matter to the vote.

While I was sorry to have to do it, I do not regret the outcome. The bill has been improved and parliament’s role enhanced. MPs of all parties have come together to do the legislative job that is one of our key functions. I am very grateful to all who supported the amendment, but particularly to those Conservative colleagues who joined with me, some of whom withstood intense pressure not to do so.

I do worry that the circumstances around the vote and its aftermath show a worrying slide towards irrationality in our political discourse. As a politician, I should expect sharp challenge from those who disagree with my decisions. But it is troubling that much of the controversy brought in allegations of an intention to sabotage Brexit that is far removed from what we were doing.

Some of this was fuelled and orchestrated by newspapers that seem entirely disinhibited in the inaccuracies they peddle and the vitriolic abuse they are prepared to heap on those who do anything they consider to be at variance with their version of what Brexit should be. This both obscures the real issues and encourages an atmosphere of crisis and confrontation between binary positions that leads directly to the death threats that we have received. In turn, this undermines the ability of politicians to engage in rational debate or make sound judgments on issues where there is often no certain answer.

I hope very much that the fallout from this episode may be beneficial. There are welcome signs from my colleagues in government of a willingness to work constructively with us and others. I have been sustained by the volume of supportive emails and letters and the willingness of many, irrespective of how they voted in the referendum, to denounce the fomenting of hatred. If parliament and government work together in their respective constitutional roles, and respect due processes, we will maximise our chances of making the right decisions as we encounter the many challenges, risks and opportunities Brexit poses for our country.

• Dominic Grieve is the Conservative MP for Beaconsfield, and a former attorney general

