Nine senior Conservatives, including two former Cabinet ministers, have put their name to cross-party amendments to the European Union withdrawal bill.

The Conservative peers, including Chris Patten, the former party chairman, and Douglas Hogg, the former agriculture minister, are supporting a series of substantial changes to the government’s flagship bill to leave the EU.

A series of amendments have been tabled, including one giving parliament a vote on staying in a customs union which is almost certain to pass, meaning MPs will get a say on the contentious issue despite the government’s efforts to kick it into the long grass.

Q&A What is a customs union? Show A customs union means that countries agree to apply no or very low tariffs to goods sold between them, and to collectively apply the same tariffs to imported goods from the rest of the world. International trade deals are then negotiated by the bloc as a whole. For the EU, this means deals are negotiated by by Brussels, although individual member state governments agree the mandate and approve the final deal. The EU has trade deals covering 69 countries, including Canada and South Korea, which the UK has been attempting to roll over into post-Brexit bilateral agreements. Proponents of an independent UK trade policy outside the EU customs union say Britain must forge its own deals if it is to take advantage of the world’s fastest-growing economies. However they have never explained why Germany manages to export more than three times the value in goods to China than Britain does, while also being in the EU customs union. Jennifer Rankin

Another significant proposal is to give Parliament a voice on what happens in the event the government’s final Brexit deal being voted down.

Ministers have warned MPs that if they fail to back it the only other option is crashing out of the EU without any deal at all. The peers, however, think that MPs should be free to consider other options.

The revolt, which was dubbed Theresa May’s Waterloo after the Duke of Wellington, whose ancestor defeated Napoleon in the 1815 battle, put his name to one of the amendments, is expected take place immediately after the Easter recess.

MPs would then get the chance to vote on the proposals, which also include removing the government’s fixed exit day and a challenge to Henry VIII powers, which critics say are a power grab by ministers.

Other Tory peers backing the cross-party amendments are Ros Altmann, the former pensions minister, and former newspaper editor Patience Wheatcroft.

Angela Smith of Basildon, the shadow leader of the Lords, said: “This bill is the first piece of legislation that reflects the practical implications of Brexit rather than the principle of whether or not we should continue to be in the EU.

“It is therefore, an opportunity for the government to take a pragmatic view of how best to protect the rights of UK citizens rather than be distracted by the ideological pursuits of some on the Commons backbenches.

“The period between now and the start of report stage offers Theresa May and

her ministers an opportunity to bring forward sensible changes in response to

concerns raised previously in the Lords.

“A failure to do so however, will amount to kicking the can down what could be a very rocky road. And our peers won’t be shy about sending amendments to the Commons, giving MPs a further chance to scrutinise the detail of the bill.”