Britain's Labour Party has shifted its policy on Brexit, saying it supports staying in the EU's Single Market and Customs Union for a transitional period of up to four years before the UK formally leaves the EU.

It also holds out the possibility of staying in a form of customs union as a possible end destination.

The Conservative Party has said they want to make their own trade deals and run their own immigration policy outside the EU's Single Market and Customs Union, yet have no border infrastructure in Ireland.

The party says it would accept the rules of the Single Market and the Customs Union for a transitional period that could last up to four years, beyond the UK's formal EU departure date of March 2019.

Significantly from an Irish perspective, it holds out the possibility of remaining in a form of Customs Union, as an end destination. 

This 'soft' Brexit strategy would open the possibility of an invisible Irish border remaining.

The twist is significant as it signals tensions in UK politics and pressure in its economy.

It comes as EU/UK formal negotiations are set to resume after the summer break.

Labour Party leader Jeremy Corbyn said he did not know how long any transitional period would last, saying it would be "as long as is necessary but make it time-limited".

Asked whether the UK would continue to accept free movement and make payments into the EU budgets after March 2019 under Labour's plans, he said: "Obviously, we have got to work out what the arrangements are during the transition period and make sure we reach agreement on that.

"Quite clearly, the priority has to be protecting jobs and also understanding the needs of EU nationals that are living here."

Mr Corbyn said he could not say "at this stage" whether Labour's plan means the UK could remain in the customs union on a permanent basis.

"We need to reach an arrangement which protects the long-term trading relationship between Britain and Europe and jobs in this country, because all the manufacturing industries in Britain rely on a supply chain all across Europe," he said.

Earlier, a Labour spokesman confirmed the party would propose the same "basic terms" as Britain's current relationship with the EU during a transition period following Brexit in 2019, and after that for all options to be open. 

He was confirming a report in Britain's Guardian newspaper in which shadow Brexit secretary Keir Starmer backed "continued membership of the EU single market beyond March 2019" in an attempt to offer a clear alternative to the Brexit currently proposed by British Prime Minister Theresa May.

"We will always put jobs and the economy first," Mr Starmer told the Guardian.

"That means remaining in a form of customs union with the EU is a possible end destination for Labour, but that must be subject to negotiations.

"It also means that Labour is flexible as to whether the benefits of the single market are retained by negotiating a new single market relationship or by working up from a bespoke trade deal," he said.

Mr Corbyn's party would also "leave open the option of the UK remaining a member of the customs union and single market for good, beyond the end of the transitional period", the paper said, but only if Labour could by then have persuaded the rest of the EU to agree to a special deal on immigration and changes to freedom of movement rules.

British Prime Minister Theresa May's attempts to settle on a position and start negotiating Brexit have been hampered by infighting in her own party following a botched early general election in June in which she lost her parliamentary majority.

Her position has been weakened and many believe she is unlikely to see out a full parliamentary term, although it is unclear who might replace her.

Meanwhile, Fianna Fáil leader Micheál Martin welcomed the decision by the British Labour Party.

In a statement, Mr Martin said: "Keeping Britain in the single market and customs union is essential for Ireland. The Government must do everything it can to help make this a reality. A hard Brexit would be disastrous for Ireland and would cause serious damage to our strong relationship with Britain.

"It is also in Ireland's interest to see the UK and Northern Ireland remain in the customs union in order to prevent the imposition of border controls on the island of Ireland."

Labour Party leader Brendan Howlin also welcomed the policy shift by Mr Corbyn's party. 

In a statement, Mr Howlin said: "The acknowledgement of the need for an extensive transition period of up to four years where the UK would stay in the single market and customs union is welcome.

"It will provide an important stimulus to the ongoing debate in the UK on what their future relationship with the EU will be. I hope the UK Government will respond favourably to this development."

Sinn Féin's Westminster Brexit spokesperson Chris Hazzard MP also welcomed the development. 

He said it would "mitigate some of the impacts of Brexit. We will be in contact with the British Labour Party on their proposals and I will be meeting with them and other parties at Westminster in the coming weeks."