British Foreign Secretary Jeremy Hunt has warned fellow Conservative MPs that trying to unseat Prime Minister Theresa May in the on-going row over Brexit risks causing "the most appalling chaos that could be immensely damaging to our national reputation".

Last week May's cabinet approved her Brexit plan but two ministers resigned in protest, and a group of right-wing MPs known as the European Reform Group began to gather letters of no confidence in her, to trigger a leadership challenge.

This process has not gone smoothly, with confusion about how many MPs have signed up, but the incident has still been embarrassing.

Speaking on a trip to Iran, Hunt warned against challenging May's leadership, saying it could damage the country and also hopes of any Brexit deal being concluded.

May has "perhaps got the most difficult job of any prime minister or president in the western world at the moment", said Hunt.

"Seeking to remove her risks the most appalling chaos that could be immensely damaging to our national reputation," he added, "but also destabilising and potentially stopping us getting through to the other side of Brexit. That is why when people think about this, people will realise that backing her is the right thing to do."

To trigger a no confidence vote, the chairman of the Conservative Party's backbench group, the 1922 Committee, must receive letters from 48 MPs, but by Monday evening just 26 names had been made public.

Those tempted to rebel must weigh up the consequences, such as the bad publicity for the party, the question of who would replace May if she were ousted, and also the dilemma that if she survived, her position would be strengthened.

May is expected to meet European Commission President Jean-Claude Juncker this week to finalize her policy before leaders of the other 27 European Union nations meet at the weekend to approve it.

The biggest challenge so far has been trying to resolve the future status of the border between Northern Ireland, part of the United Kingdom, and the Republic of Ireland, an EU member state .Under those circumstances, May will have been particularly pleased to receive the support of Irish Foreign Minister Simon Coveney.

"People have underestimated her over and over again; they're wrong to. She is a very steely, determined person," Hunt said. "She has got a good deal for Britain, for the United Kingdom as a whole I should say, and she has followed through also on her commitments to Ireland and the EU."

France and Spain could be less charitable, though. The French government has warned it will be "very vigilant" about the implementation of the UK's withdrawal agreement, and wants to ensure tough safeguards are kept in place on topics such as environmental protection and competition, in particular French fishermen's access to UK waters.

Spain will also be keen to see what is said about the future status of Gibraltar, the British territory on the tip of southern Spain, in regard to any future trade agreements between the UK and the EU.