It is not possible that the European Union will offer Britain a long delay to Brexit because of upcoming European elections, British trade minister Liam Fox said on Sunday.

Fox said it was still "entirely possible" Britain leaves the bloc as scheduled on March 29 but an extension to the Article 50 negotiating period may be necessary in order to deliver a smooth exit from the bloc.

Prime Minister Theresa May has said any extension should not be beyond the end of June.

Asked how he would react if the EU insisted on a much longer delay of 21 months or two years, Fox told BBC TV: "I would be shocked because I think it is actually not a possible outcome."

"The European Union does not want Britain to fight the European elections."

Irish Prime Minister Leo Varadkar has told Cabinet colleagues that a delay of Britain's exit date from the European Union from March 29 to June is "very likely", Ireland's Sunday Independent quoted an unnamed minister as saying.

"The Taoiseach (prime minister) has privately said to us that it is very likely there will be an extension until June," the minister was quoted as saying.

An Irish government spokesman declined to comment on the veracity of the report but said the Cabinet's focus was "solely on securing a deal" and pointed to a comment by Varadkar on Friday in which he said a British exit on March 29 without either a deal or an extension was unlikely.

The Sunday Times newspaper reports that Brexit-supporting MPs who voted down Prime Minister Theresa May's European Union withdrawal deal in January have outlined demands for a revised treaty to ensure their support.

MPs overwhelmingly rejected May's deal in January. Many were unhappy with the "Irish backstop", insurance to prevent return of hard border controls between EU member Ireland and British-ruled Northern Ireland.

Critics said it could leave the country tied to EU rules indefinitely. Britain, due to exit the bloc on March 29, is attempting to amend the deal to provide assurances that the backstop would not be indefinite.

The Sunday Times said hardline Brexit supporters from May's Conservative Party had drawn up a document outlining three tests the deal must pass to gain their support.

These are a "clearly worded, legally binding, treaty-level clause which unambiguously overrides" the text of the withdrawal agreement, with language that goes beyond emphasising the temporary nature of the backstop and a clear means to exit the backstop if subsequent trade talks fail.

The paper said the plan had been drawn up with the support of the Democratic Unionist Party, the Northern Irish party which props up May's minority government.

If May secures the demands, she would win the backing of the DUP and the Brexit-supporting MPs in a vote on the deal which she has promised would be held before March 12, the Times said.

The EU's chief negotiator Michel Barnier said on Friday that the bloc was ready to give Britain more guarantees that the backstop was only intended to be temporary and used for a "worst-case scenario".

"We know that there are misgivings in Britain that the backstop could keep Britain forever connected to the EU," Michel Barnier said in an interview with Germany's Die Welt newspaper.

"This is not the case. And we are ready to give further guarantees, assurances and clarifications that the backstop should only be temporary."