Labour peers in the House of Lords are preparing for another Brexit showdown with Theresa May.

Labour and anti-Brexit campaigners believe the UK government can be defeated on the Trade Bill.

They want to force May's government into negotiating a customs union with the European Union.

Meanwhile, pro-EU MPs believe they will soon have the numbers to force May into a soft Brexit

LONDON — Labour peers in the House of Lords are preparing for another Brexit showdown after Parliament returns from summer holidays amid a feeling in Westminster that the biggest "pro-Remain" rebellions are yet to come.

Labour whips in the Lords believe that the second reading of the Trade Bill on Tuesday, September 11 will be the start of "long run" campaign to force Theresa May into negotiating a customs union with the European Union.

The UK government's current policy is to leave the customs union. However, doing so will create new border checks for British exporters, resulting in costly delays in cross-border trade. The policy also threatens the open border on the island of Ireland, which May has promised to preserve no matter what happens in Brexit talks.

Despite having numerous Brexit amendments thrown out by MPs, Labour peers are in no mood to back down, and are working together with Liberal Democrat and cross-bench peers to inflict another defeat on the government.

"There is still a huge appetite to force the government's hand. We are all ready to go again," a senior Labour source in the Lords said. "There is no fatigue. We are used to things getting overturned. It doesn't put us off."

Labour whips in the House of Lords had also discussed tabling amendments to the Taxation Bill, which will be sent back to peers on the first day back from recess on Tuesday, September 4th.

However, May's government quietly added "aids and supplies" language to the bill prior to the summer recess, an archaic convention which instructs the House of Lords not to amend the legislation.

Labour peers are planning to submit a regret motion to the Taxation Bill in protest against the government's move to withhold it from parliamentary scrutiny and prevent another customs union amendment tabled.

The Trade Bill will be "where all the action happens," a Labour source told BI. Anti-Brexit campaigners also believe this piece of legislation represents the best chance of forcing the government into a softer Brexit position.

Labour peer Andrew Adonis told BI: "Fiddling with 101 variants of a customs union is never going to work. Not least because the Brexiteers are against the principle of customs unions anyway."

Back in the House of Commons, pro-EU MPs believe a majority of MPs are ready to sign off any new House of Lords attempt to force Prime Minister May into negotiating a customs union with the EU.

Tory "rebel" MPs believe that anti-hard Brexit colleagues who have up to now been reluctant to vote against May on Brexit legislation are ready to rebel this autumn if a deal is no closer to being agreed.

A significant number of Conservative MPs have stayed loyal in order to "let the prime minister have some space to negotiate," one leading Tory rebel told BI, but are increasingly anxious about the growing prospect of no deal.

Another leading Conservative backbencher, Stephen Hammond, made this claim in an interview with BI last month.

"There are a number of colleagues who have wanted to give the prime minister room and scope to negotiate what her desired outcome is," the former government minister said.

"However, they are also colleagues who are clear that if the outcome is a no deal, then they'll make their view well-known that this isn't acceptable for Britain. No deal is not the default. That will not happen."

A prominent Labour backbencher echoed this, telling BI this week that pro-EU MPs were "really encouraged" by how many MPs voted for a customs union before the summer recess, and expect more to follow in future votes.