Political jostling continued in Westminster on Monday night as British Prime Minister Theresa May switched attention to "selling" her proposed Brexit agreement to the business world.

May addressed 2,000 top business leaders at the annual conference in London of the Confederation of British Industry (CBI).

Hours later May's opposite number in Westminster, Labour leader Jeremy Corbyn faced the same audience.

In Europe's mainland, trouble was brewing for the Brexit negotiations with media reports in London saying the Spanish government has threatened to reject May's Brexit deal over the issue of Gibraltar.

The Spanish government wants last-minute changes be made to the text of the plan ahead of a crunch summit of EU leaders in Brussels on Sunday.

Spain has said it wants clarity over the status of Gibraltar before it would support the Brexit agreement.

The Independent in London said the Brexit withdrawal agreement has to be approved by qualified majority voting at the European Council this Sunday, meaning that if other countries add their voices of dissent to Spain's, the agreement could be in trouble.

The report added that the EU has so far publicly stressed its unity in dealings with Britain, though it is understood that some other countries have problems with the future relationship proposals, fearing too many concessions may have been granted to May's government.

The Independent said countries understood to have voiced concerns in private include Denmark, Belgium and the Netherlands, who were annoyed that Britain has been given an EU customs union as part of a backstop arrangement without a formal agreement guaranteeing their retention of fishing rights in British waters.

Downing Street on Monday night ruled out any kind of concessions,with a spokesperson saying: "The draft withdrawal agreement agreed last week covers Gibraltar. The prime minister has been clear that we will not exclude Gibraltar or the other overseas territories or the Crown dependencies from our negotiations on the future relationship. We will get a deal that works for the whole UK family."

In Westminster, attention switched to whether the number of names needed to force a leadership challenge had been reached.

Although more named were added to the list Monday, it appears the required number has yet to reach 48 from Conservative MPs expressing no confidence in May.

In his speech to the CBI, Corbyn said Labour would vote against May's Brexit deal, adding his party would not countenance a no deal Brexit.

Corbyn added: "If the government cannot get its central policy through parliament, then we will demand a general election. But if we cannot secure a general election, we have been clear that all options must remain on the table, including a public vote."

In her speech to the CBI, May said she was not willing to reopen discussions with Brussels over the withdrawal agreement, adding she expected to hammer out a framework for a future trade relationship this week, before signing off the deal on Sunday at the EU council meeting when the 27 leaders gather in Brussels.

After the speeches by the two political heavyweights, the CBI's Director General Carolyn Fairbairn gave her response, which indicated May came out on top.

Fairbairn said the prime minister had opened the door for enterprise and government to work together and build a fairer and more competitive Britain.

"Getting the right Brexit deal will dominate the coming weeks but we can't afford to lose sight of the domestic agenda. At the heart of that is people. We need a focus on jobs, skills and investment.

"The prime minister has opened the door for enterprise and government to work together and build a fairer and more competitive UK," added Fairbairn.