Union Flags and European Union flags fly near the Elizabeth Tower, housing the Big Ben bell, in Parliament Square in central London, Britain September 9, 2017. [Photo/Agencies]

LONDON - Prime Minister Theresa May appointed the top official at the Brexit ministry as her EU adviser on Monday, taking tighter control of divorce negotiations just as Britain tries to shift the focus of talks onto the future relationship with Europe.

"In order to strengthen cross-government co-ordination of the next phase of negotiations with the European Union, the Prime Minister has appointed Oliver Robbins as her EU Adviser in the Cabinet Office, in addition to his role as EU Sherpa," a government spokesman said in a statement.

Robbins will continue to lead British officials in the negotiations and will work closely with Brexit Secretary David Davis, the spokesman said.

Robbins, 42, who media reported had sometimes clashed with Davis, had worked in the Brexit ministry since shortly after it was set up following the EU referendum in June 2016.

May, whose position was weakened after losing her governing Conservative Party's majority at a June election, has been criticised for failing to give clear instructions to her negotiating team.

May dismissed the suggestion that the appointment showed the initial negotiating set-up had been flawed.

May said the appointment was a sign "that the negotiations are getting into a more detailed and more intense phase. And as a result, I think it's right Ollie Robbins concentrates on that," she told reporters in the Canadian capital Ottawa on Monday after talks with Prime Minister Justin Trudeau.

Keir Starmer, Brexit spokesman for the opposition Labour Party, said "moving key individuals at this critical time adds a whole new dimension to government's chaotic approach to Brexit".

Foreign Minister Boris Johnson was accused by colleagues on Sunday of "backseat driving" after setting out his own vision for Brexit days before May is due to give a major speech on the subject on Sept 22, ahead of the next round of talks.

"The UK government is driven from the front and we all have the same destination in our sights," May said in Ottawa when asked about Johnson's comments.

So far negotiations on the terms of the divorce have made limited progress, prompting warnings from the EU the start of discussions on post-Brexit ties could be pushed back from October.

"The negotiations are being conducted in a constructive and positive spirit and we are making progress on the areas that we've been addressing so far," said May.