Today I will be speaking at the launch of a campaign in the House of Commons that is vital to the future of our country.

It is a campaign that is not about Brexit. It is not about our struggling health service or our underfunded schools. It is not about building more houses or reforming social care and yet it is vital for all these things. It is a campaign to save our parliamentary democracy. I will give it my wholehearted support, and I urge you to do so too.

The government is already using the EU (withdrawal) bill to grant itself new powers to bypass parliament. Now the recently published trade bill has compounded this attempt to undermine the capacity of MPs to hold the executive to account and scrutinise legislation. It will sign us up to a host of treaty obligations that parliament will be powerless to oppose.

I seem to remember that we were told that Brexit was about bringing sovereign powers back to the UK parliament – not to hand them over to Theresa May and the executive. And I am absolutely sure that no one voted to hand over those powers to her trade secretary, Liam Fox.

The government’s trade bill is supposed to set the legislative framework according to which all our future trade agreements with other countries will be scrutinised and rendered democratically accountable in parliament. The Institute for Government recommended that the bill should include a guarantee that all future trade deals would be granted a full parliamentary debate and a vote.

Yet the government has ignored this and every other recommendation to ensure proper parliamentary scrutiny of our trade relations with other states. Instead it proposes to roll over existing EU trade deals even when they have not been ratified, and to smuggle through future deals via a procedure that is an affront to democracy.

Under an archaic gentleman’s agreement called the Ponsonby rule that dates back to 1924, the government will be obliged only to lay the text of a new trade treaty before parliament for 21 sitting days before it passes into law automatically. No need for a debate. Certainly no need for a vote.

The government has confirmed that it wants to use this procedure to ratify future trade deals after we leave the EU, as provided for by the Constitutional Reform and Governance Act 2010.

The only way for MPs to object to a trade agreement during the 21-day period is via an opposition day debate. Yet there is no need for the government to grant such a debate. In fact, the current government refused to schedule a single opposition day debate between February and September this year.

And even if we get a debate and win it, the government does not have to change tack. Under this absurd rule, it only has to write a letter explaining why it disagrees with parliament and insist on its own way.

Labour is calling for a thorough overhaul of this procedure. Treaties negotiated with foreign powers create binding obligations on future generations that cannot be repealed in the way that domestic law can. As a consequence, the most rigorous process should be in place to scrutinise such treaties before they ever come to be ratified.

Labour believes that every trade deal should come before parliament for a full debate on the floor of the House of Commons, with a vote at the end of that debate. There should also be a new committee with scrutiny powers similar to those currently enjoyed by the European scrutiny committee, which has been able to call the government to account over every trade deal signed at the European level.

MPs should be able to debate, amend and approve a mandate for the negotiation of any trade agreement before talks start, based on an independent impact assessment of what social, economic and environmental risks might be expected. And MPs should have access to the consolidated texts of those agreements as they are being formulated, as a basic right of access to the information that they need to do their job of representing their constituents and holding the government to account.

This government has a particular aversion to transparency. It was only after months of protest by me and other Labour politicians that Liam Fox finally opened a reading room for MPs to even be allowed to see the text of the controversial Transatlantic Trade and Investment Partnership (TTIP) between the EU and US.

I was the first MP to use the reading room, and I had to hand over my mobile phone and any other recording device as a condition of entry. I was forced to sign a confidentiality clause prohibiting me from sharing any of the details that I read with anyone else. To add insult to injury, the government had stalled for so long that the TTIP negotiations were already over by the time I was given access to the room.

More than 120 MPs from across all six opposition parties that sit in Westminster have signed the two early day motions calling for the government to respect parliamentary scrutiny of trade deals. This makes them together the second most widely supported of all 550 early day motions currently before parliament.

The campaign I will help launch this week is backed by the public sector trade union Unison as well as Global Justice Now, War on Want and other members of the Trade Justice Movement. It seeks to restore parliament’s sovereign right to decide who we conclude special trade agreements with in the future, and on what terms.

Today’s trade agreements touch on so many social and economic aspects of our common life, as well as the protections we afford the natural world. It would be a savage irony if Theresa May’s final political epitaph were to be: “She returned powers to the UK only to leave us with even less democratic oversight than we had in the EU.”

• Barry Gardiner is shadow secretary of state for international trade, energy and climate change. He has been Labour MP for Brent North since 1997