British Prime Minister Theresa May has insisted she had the "determination to deliver Brexit" as she came under pressure from both wings of the Tory party to change course.

Eurosceptics have urged Mrs May to drop the proposal for a "customs partnership" which they fear would tie the UK too closely to Brussels.

But pro-EU Tories have pushed for her to abandon red lines and keep the UK in the single market, claiming she would have cross-party support in the House of Commons for such a move.

Mrs May said she had an "absolute determination to make a success of Brexit, by leaving the single market and customs union and building a new relationship with EU partners that takes back control of our borders, our laws and our money".

She said the UK was "making good progress towards that goal and we will carry on doing so with resolution in the months ahead".

A decision on the British government's preferred customs option has been postponed after Mrs May's Brexit "war cabinet" failed to reach agreement.

A number of cabinet ministers spoke out against her hybrid "customs partnership" model which would see Britain collect tariffs on behalf of the EU for goods destined for the block, with firms potentially claiming back a rebate if products remained in the UK on a lower-tariff regime.

The Sunday Telegraph quoted a cabinet source saying it would be "unimaginable for the Prime Minister to press on with the hybrid model after it has been torn apart by members of her own Brexit committee".

But pro-EU Tories, seeking to capitalise on the lack of agreement in the Cabinet and a House of Lords victory on the customs union, pushed for Mrs May to abandon her Brexit strategy and instead commit to a Norway-style approach within the European Economic Area (EEA) and European Free Trade Association (Efta).

Such a move would be unacceptable to many Brexit supporters because it would leave the UK expected to accept free movement, although its advocates claim Mrs May would have some ability to impose conditions.

A House of Lords amendment which would require the government to negotiate continued membership of the EEA could be put to a vote on Tuesday, but Labour peers have reportedly been told to abstain.

Foster says EU and UK must work together

Meanwhile, DUP Leader Arlene Foster has said she wants the EU to work together with the British government to find a solution that would neither result in a return of a hard border nor isolate Northern Ireland from the rest of the UK.

Speaking on the BBC's Andrew Marr show, Ms Foster said she would like to see more pragmatism from the EU over future arrangements after Brexit.

Ms Foster also said she is waiting to hear the British government’s plans to deal with future customs arrangements with the European Union.

Ms Foster said she does not believe Northern Ireland has to stay in the customs union to have free-flow across the border.

"I think it is very clear that the British people voted to leave the customs union, that is our position as well," she said.

She added: "We do not believe that we have to stay in the customs union to have free flow between ourselves and the Republic of Ireland."

"We believe that there are ways to deal with this and indeed back in August last year the government put forward various proposals - we were disappointed there was not the engagement from the European Union.

"What we would like to see from the EU is less rhetoric and more engagement in relation to the pragmatic way forward," she said.