With less than a year to go until the UK officially exits the EU, Theresa May faces a field full of Parliamentary hurdles.

Last week the PM’s plan for a “customs partnership” was thrown into doubt, and on Tuesday evening the House of Lords will debate three cross-party amendments to the government’s Brexit bill on issues including Britain’s membership of the single market. 

Peers inflicted an embarrassing defeat on the government last month after they voted for the UK to remain part of a customs union with the EU and for protecting citizens’ rights post-March 2019. 

Brexiteer heavyweights, including including former Chancellors Lord Lamont and Lawson, argued that the amendments would “undermine the government’s negotiating stance”.

Leave-supporting MPs, including Jacob Rees-Mogg and Daniel Kawczynski, warned that a risky strategy was being played by the Lords in trying to “block Brexit”.

But pro-Europe pressure group Open Britain was quick to point out five occasions on which Brexit campaigners stressed the UK’s departure from Europe was rooted in a need for its Houses of Parliament to make its own laws and legislation – a process involving both the Commons and Lords.

1. Nigel Farage

“I believe in Britain. A Britain outside of the EU with an exciting future ahead of it.  A proud, patriotic country that has control of its borders, represents itself on the world stage and makes its own laws in our own sovereign Parliament.”

2. Michael Gove

“I believe – as a matter of deep principle – in Parliamentary democracy. I believe that the laws which govern us all should be made by politicians accountable to the people. I believe that our membership of the European Union undermines that precious principle.” 

3. Boris Johnson

“We have given so much to the world, in ideas and culture, but the most valuable British export and the one for which we are most famous is the one that is now increasingly in question: Parliamentary democracy – the way the people express their power.” 

4. Daniel Hannan

“There are several ways to square the circle. For example, having taken back Parliamentary sovereignty, the UK could agree to continue with some of its existing programmes and obligations through bilateral treaties, either open-endedly or for a guaranteed period of time.”

5. Leave.EU

“Vote to leave the EU to ensure law-making power returns to our sovereign national Parliament.”

James McGrory, executive director of Open Britain, said: “Leave campaigners used to pretend Brexit was all about ‘restoring parliamentary sovereignty’, despite the fact we never lost it when we were in the EU.

“But now that Parliament is exerting its sovereignty to push back against the government’s destructive Brexit plans, they immediately cry foul.

“Brexit will affect the future of this country for decades to come.  It is too big an issue for Parliament to ignore and MPs would not be doing their jobs properly if they just rubber-stamp whatever deal the government comes back with.”

He added: “The role of Parliament is of crucial importance to this process, but we also need a People’s Vote on the terms of Brexit, so that the people can have their say on the best way forward for the country.”

Jayne Adye, director of cross-party pro-Brexit campaign Get Britain Out, said the public want the referendum decision to be respected.

“Elected representatives in the House of Commons voted overwhelmingly to delegate this decision to the electorate,” she told HuffPost UK.

“Having given their verdict, the Great British Public now expect their instruction to be implemented.

“The Lords are making a mockery of the Salisbury Convention, where they are to refrain from wrecking the manifesto policies of an elected government. The Conservative manifesto was clear – a pledge to leave the single market and customs union.”

Adye said no Brexiteer had ever claimed the UK’s parliamentary institutions were “perfect”.

“We have a situation whereby the Lords are disregarding establishment constitutional protocols,” she added. “This is driven by political calculation and signifies a dangerous divergence from the foundations of our parliamentary system.

“17.4 million Britons voted to Get Britain Out of the EU. For this to be undermined at the behest of a few hundred unelected peers would greatly undermine public trust in the parliamentary system”.

Labour MPs Chuka Umunna and Alison McGovern have urged peers to come out in favour of the UK remaining in the single market, or European Economic Area (EEA), despite their party whipping against it.  The result is expected late on Tuesday evening.

“We know the damage leaving the single market will do to our economy, to public services and to our NHS, so it would go against Labour’s progressive values for the party not to vote in favour of these amendments,” Umunna said.

McGovern, who co-chairs the Labour Campaign for the Single Market, added: “The government’s own analysis shows that crashing out of the single market will cause a crippling hit to our economy, which is why it made no sense for Theresa May to rule out single market membership before the Brexit negotiations had even started.

“If passed, these amendments in the Lords would be a big step forward in avoiding a hard and destructive Brexit, which is why all Labour peers should get behind them.”