Labour is ready to team up with Tory rebels to force Theresa May to agree a customs union with the EU, Sir Keir Starmer has signalled.

The Brexit Secretary said "crunch time is coming for the Prime Minister" as he claimed Ms May did not have enough support in Parliament to push through plans to leave the vital EU trade agreements.

Ahead of a major Brexit speech by Jeremy Corbyn on Monday, Sir Keir confirmed the party had "unanimously agreed" to back staying in a customs union with the EU after Brexit.

He told the BBC's Andrew Marr Show: "The customs arrangements at the moment are hard-wired into the membership treaty so I think everyone now recognises there has to be a new treaty. It will do the work of the customs union so it's a customs union."

Sir Keir indicated that Labour would support rebel amendments to a key Brexit bill tabled by pro-EU Tories Anna Soubry and Ken Clarke, which would put customs union membership back on the table.

He said: "The Labour front bench put down a number of amendments paving the way for the option of a customs union - they went down a few weeks ago.

"Now these cross-party amendments have gone down essentially saying the same thing and to put it bluntly crunch time is coming for the Prime Minister."

Asked whether Labour would back the cross-party amendments, he said: "We haven't made a final decision on that but they are so close to our amendments ... but whether it's our amendments or cross-bench amendments, crunch time is now coming for the Prime Minister because the majority of Parliament does not back her approach to a customs union.

"The majority in Parliament needs to be heard and it will be heard sooner rather than later."

The amendment to Ms May's trade bill is shaping up to be a crucial flash point, as a growing number of Tory MPs are likely to support remaining in a customs union with the EU.

It is also backed by the small group of Tory "mutineers" who inflicted an embarrassing defeat on the Government last year by winning a meaningful vote for MPs on the final deal.

Eurosceptic cabinet minister Liam Fox urged Tory Remainers to keep an "open mind" as he suggested Ms May's Brexit war cabinet on Thursday has agreed proposals that would deal with their fears.

The International Trade Secretary told The Andrew Marr Show: "I would say to my colleagues that Theresa May has kept a broad range of views on the European issue for a reason.

"We sat down with those differing views, we set out the issues, we looked at the options and we came to an agreement that we are all happy with."

He added: "I hope that they will have an open mind and listen to what the Prime Minister says because I think that what the Prime Minister will set out will deal with a lot of the reservations that they have."

The biggest issues facing UK on leaving EU Show all 8 1 /8 The biggest issues facing UK on leaving EU The biggest issues facing UK on leaving EU Post-Brexit immigration workers sorting radishes on a production line at a farm in Norfolk. One possible post-Brexit immigration scheme could struggle to channel workers towards less attractive roles - while another may heighten the risk of labour exploitation, a new report warns. PA The biggest issues facing UK on leaving EU Customs union A key point in the negotiations remains Britain's access to, or withdrawal from, the EU customs union. Since the referendum there has been hot debate over the meaning of Brexit: would it entail a full withdrawal from the existing agreement, known as hard Brexit, or the soft version in which we would remain part of a common customs area for most goods, as Turkey does? No 10 has so far insisted that “Brexit means Brexit” and that Britain will be leaving the customs union, but may be inclined to change its position once the potential risks to the UK’s economic outlook become clearer. Alamy The biggest issues facing UK on leaving EU Northern Ireland-Irish border Though progress was made last year, there has still been no solid agreement on whether there should be a hard border between Northern Ireland and the Republic of Ireland. To ensure borderless travel on the island, the countries must be in regulatory alignment and therefore adhere to the same rules as the customs union. In December, the Conservative Party’s coalition partners, the DUP, refused a draft agreement that would place the UK/EU border in the Irish Sea due to its potential to undermine the union. May has promised that would not be the case and has suggested that a “specific solution” would need to be found. Getty The biggest issues facing UK on leaving EU Transition period Despite protests from a small number of Conservative MPs, the Government and the EU are largely in agreement that a transitional period is needed after Brexit. The talks, however, have reached an impasse. Though May has agreed that the UK will continue to contribute to the EU budget until 2021, the PM wants to be able to select which laws made during this time the UK will have to adhere to. Chief negotiator Michel Barnier has said the UK must adopt all of the laws passed during the transition, without any input from British ministers or MEPs. EPA The biggest issues facing UK on leaving EU Rights of EU citizens living the UK The Prime Minister has promised EU citizens already living in the UK the right to live and work here after Brexit, but the rights of those who arrive after Brexit day remains unclear. May insists that those who arrive during the transition period should not be allowed to stay, whereas the EU believe the cut-off point should be later. Getty The biggest issues facing UK on leaving EU Future trade agreement (with the EU) Despite this being a key issue in negotiations, the Government has yet to lay out exactly what it wants from a trade deal with the EU. Infighting within the Cabinet has prevented a solid position from being reached, with some MPs content that "no deal is better than a bad deal" while others rally behind single market access. The EU has already confirmed that access to the single market would be impossible without the UK remaining in the customs union. Getty The biggest issues facing UK on leaving EU Future trade agreements (internationally) The Government has already begun trying to woo foreign leaders into prospective trade agreements, with various high profile state visits to China, India and Canada for May, and the now infamous invitation to US President Donald Trump to visit London. However the UK cannot make trade agreements with another country while it is still a member of the EU, and the potential loss of trade with the world's major powers is a source of anxiety for the PM. The EU has said the UK cannot secure trade deals during the transition period. EPA The biggest issues facing UK on leaving EU Financial services Banks in the UK will be hit hard regardless of the Brexit outcome. The EU has refused to give British banks passporting rights to trade within the EU, dashing hopes of a special City deal. However according to new reports Germany has suggested allowing trade on the condition that the UK continues paying into the EU budget even after the transition period. Getty

The next stage of the bill has been pushed back until April, partly due to attempts by government whips to manage the rebellion.

Asked if legislation was being delayed because the Government would lose, Dr Fox replied: "We want to persuade our colleagues of the merits of our argument before we take the bill forward."