Image caption Sir Keir Starmer met traders on both sides of the Irish border on Monday

The only way to avoid a hard border is for the UK to remain in the customs union, the shadow Brexit Secretary has said.

Sir Keir Starmer said a customs union should have been a negotiating option.

However, he ruled out supporting a second referendum during his visit to Londonderry on Monday.

Sir Keir fielded questions from business leaders before visiting a number of companies on both sides of the border in the afternoon.

Referring to a customs union, he said Labour had always called for this option to be kept.

"It should be a negotiating option," he said. "Obviously, there are lots of benefits from a customs union, none more so, in many respects, than in Northern Ireland."

Speaking earlier, ahead of his visit, Sir Keir said Labour would not support a deal to leave the EU if it "does not work for all communities in Northern Ireland".

He said there must be "no rowing back" from the commitments the Conservative government made to avoid a hard border.

Its pledge came at the end of the first phase of Brexit talks in December.

A so-called "hard border" is one that would see the return of physical customs posts and inspections along the 310 mile (500km) frontier between Northern Ireland and the Republic of Ireland.

'Locked-in'

Writing in the Belfast Telegraph on Monday, Sir Keir said the government had made "a number of solemn commitments to Northern Ireland" during last month's talks.

"None more important than the commitments to protect "north-south cooperation" and "the avoidance of a hard border, including any physical infrastructure or related checks and controls," he wrote.

"Those commitments were made 'in all circumstances'. That means there can be no rowing back.

"They are locked-in, even in the event of a no deal."

Image copyright AFP Image caption An agreement on phase one of the Brexit negotiations between the EU and UK was reached on 8 December

Sir Keir claimed that a "bitter divide" within the Conservative government over Brexit policy was the "the single biggest threat" to securing a deal with the EU.

As well as meeting cross-border traders during his trip, the shadow Brexit secretary also met Northern Ireland's political leaders as talks aimed at restoring devolution continue at Stormont.

Analysis: BBC NI political correspondent, Enda Mc Clafferty

Sir Keir Starmer is no stranger to Northern Ireland having previously worked as a human rights advisor to the Policing Board.

It is through that link that he accepted an invitation from a former vice-chair of the board, Denis Bradley, to visit the north west.

The shadow Brexit secretary addressed business leaders at a breakfast gathering in Derry before travelling to the border to hear at first hand from companies who trade on both sides.

As someone who wants the UK to remain in the customs union after Brexit, which would avoid a hard border, Sir Keir was likely to receive a warm reception.

But among politicians, there is no agreement on what is best for Northern Ireland post-Brexit.

In his newspaper article, Sir Keir accused Prime Minister Theresa May of saying "one thing to the EU and another to her MPs" regarding the transition period between March 2019 and the UK's final exit deal with the EU.

"That is because of the continuing bitter divide in the Conservative Party over Brexit; a divide that still exists around the Cabinet table," he wrote.

Labour wants the UK to remain in the single market and the customs union during the transitional period.

Sir Keir wrote that as well as providing certainty for businesses, this would "provide the time and space to address the complex questions that arise in Northern Ireland".

'Cliff-edge Brexit'

He warned Mrs May that the "clock is ticking" on the future of customs arrangements, ahead of the next phase in the Brexit negotiations.

"The Prime Minister has just eight weeks to face down the divisions within her party and agree the necessary transitional arrangements by the time the EU Council meets in March.

"Every day she delays, she increases the uncertainty and risk of a cliff-edge Brexit."

The joint EU-UK document agreed on 8 December said any future deal must protect "North-South co-operation" and hold to the UK's "guarantee of avoiding a hard border".

It also said "no new regulatory barriers" will be allowed between Northern Ireland and the rest of the UK.

Over the past few days, a number of backbench Conservative MPs have criticised the government's approach to its latest negotiations with the EU.

Media playback is unsupported on your device Media caption Theresa Villiers MP: "If you go too far with compromise... we wouldn't genuinely be leaving"

The MPs included former Northern Ireland Secretary Theresa Villiers, who warned that the UK could sign up to a deal that would "keep us in the EU in all but name".

She told the BBC's Sunday Politics: "There is only so far you can go with compromise without ultimately finding yourself in a position where you're selling out all the people who voted to leave."