The European Parliament's Brexit steering committee warned Wednesday that Britain's leadership election had increased the risk of a no-deal Brexit.

Former Belgian premier Guy Verhoftstadt's committee met EU Brexit negotiator Michel Barnier just as ardent Brexiteer Boris Johnson was preparing to move into Downing Street.

They stuck to Brussels' line that the withdrawal agreement signed by Johnson's predecessor will not be renegotiated.

But they reiterated that they would support EU leaders if they decide to rewrite an accompanying non-binding political declaration to seek "a more ambitious future EU-UK partnership".

A closer future relation could ensure "the Irish backstop would not be necessary". Johnson insists the backstop must be removed from the withdrawal agreement, an idea rejected by EU capitals.

The group congratulated Johnson, but warned that recent statements, not least those made during the Conservative Party leadership campaign, have greatly increased the risk of a disorderly exit of the UK.

And they warned that a disorderly or no-deal Brexit - which Johnson has said he will pursue if the deal is not rewritten before October 31 - would be "very damaging".

"Such an exit will not be mitigated by any form of arrangements or mini deals between the EU and the UK."

The Brexit Steering Committee "recalls that there is no transition period without a withdrawal agreement," it said.

Johnson takes charge as Britain's prime minister on Wednesday, on a mission to deliver Brexit by October 31 with or without a deal.

A politician with celebrity status best known for his gaffes and rhetorical flourishes, Johnson will be appointed by Queen Elizabeth II at Buckingham Palace.

The 55-year-old will then deliver a speech outside his new Downing Street home, setting out the tone for his premiership.

The former foreign secretary became the new leader of the governing, centre-right Conservative Party on Tuesday.