* Top Brexit official moves to PM May's team
* Boris piles pressure on May ahead of major speech
* May tightens control over Brexit talks
* Opposition say May's government in chaos over Brexit (Adds reaction)
LONDON, Sept 18 (Reuters) - 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.
Robbins will be replaced as permanent secretary of the Brexit department by his second-in-command Philip Rycroft.
May, whose position was weakened after losing her governing Conservative Party's majority at a June election, has been criticized for failing to give clear instructions to her negotiating team.
Her 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.
"Moving key individuals at this critical time adds a whole new dimension to government's chaotic approach to Brexit," opposition Labour Party Brexit spokesman Keir Starmer said.
"The real problem we have is the failure of the government to confront the difficult policy choices it faces," Bob Kerslake, a former top government official told BBC Radio, adding the timing of the move was odd.
"It could be ... a simple case of the prime minister wanting more control and influence over the process, it could be down to clash of personalities," he said. "What I worry about is the signal it sends ... about our organization and capability in this crucial time."
May is due to make a speech on Britain's future EU relationship ahead of the next round of talks later this month.
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. (Additional reporting by Elizabeth Piper and Estelle Shirbon; Editing by Guy Faulconbridge and Janet Lawrence)