Senior British ministers have agreed to offer more money to Brussels in Brexit negotiations, but only as part of a final deal on leaving the EU, a government source said Tuesday (Nov 21).

Leading eurosceptics were among ministers who signed off on the idea at a Cabinet sub-committee meeting late Monday in a bid to move the withdrawal negotiations onto trade.

The source told AFP that "no numbers" were discussed, and stressed that the agreement to up the financial offer was dependent on a wider deal on future relations being struck.

A Downing Street source said: "It remains our position that nothing's agreed until everything's agreed in negotiations with the EU. The UK and the EU should step forward together."

Britain wants EU leaders meeting in December to agree that "sufficient progress" has been made on the financial settlement, the issue of Ireland and EU citizens' rights, to move the talks onto a future EU-UK trade deal.

Prime Minister Theresa May has promised that no European Union member state would have to pay more because of Britain's exit.

British media reports suggest the government could double this to €40 billion - although that would still fall short of EU estimates of around €60 billion.

May has to tread carefully, as there is strong opposition from her backbench Conservative MPs to handing over a large sum, even if Brussels says the money is simply Britain's share of four decades of EU membership.

Former Conservative minister Robert Halfon said the public would go "bananas" if Britain committed huge sums at a time of tightened public spending.

Former Brexit minister David Jones, who left May's Cabinet in the summer, confirmed at a conference in London on Tuesday that the new financial offer would be linked to moving the talks forward.

"As I understand it, it will be made absolutely clear by the government that the payment of any further sums of that nature will be subject to there being an agreement as to our future relationship," he said.

However, Irish Foreign Minister Simon Coveney warned his government would not give up its concerns about the border with Northern Ireland, which risk holding up the negotiations.

"Anybody who thinks that just because the financial settlement issue gets resolved... that somehow Ireland will have a hand put on the shoulder and be told, 'Look, it's time to move on.' Well, we're not going to move on," he told London's Evening Standard.

Jones, a strong supporter of Brexit, said the talks should be suspended unless Brussels gives ground.

"We are arriving at the point where the EU is not really negotiating seriously and we should therefore suspend our participation in the negotiations and tell them, when you are willing to talk sensibly then we'll come back," he said.

Speaking at the same conference, Brexit Secretary David Davis repeated that Britain was planning for "every eventuality" but said he expected the two sides to reach a deal. afp