* Siemens UK CEO says Brexit remains unclear
* Siemens boss says UK should stay in customs union
* Siemens boss: 'no-deal Brexit' will not be pleasant
LONDON, June 20 (Reuters) - The United Kingdom needs to urgently give clarity on how trade with the European Union will work after Brexit and the world's fifth largest economy should remain in the customs union unless there is a proper alternative, the boss of Siemens UK said.
With nine months until the United Kingdom is due to leave the EU on March 29, little is clear about how trade will flow as Prime Minister Theresa May, who is grappling with a rebellion in her party, is still trying to strike a deal with the bloc.
"My biggest worry about Brexit is that I don't know what we are planning for," Juergen Maier, 54, the UK CEO of Siemens told Reuters in an interview.
"We need to put something in place quickly that works and if that is not possible, and until that point, then we have to just default to staying in the customs union," Maier said.
The UK has ruled out staying in the EU customs union, which clubs 28 EU members in a duty-free area where there is a common import tariff for non-EU goods.
But the nature of its future trading relationship with the world’s biggest trading bloc has split May’s government and Conservative Party.
Under the current timetable, both London and Brussels are working towards a final Brexit deal in October to give enough time to ratify it by Brexit day in March 2019.
"We ship thousands of goods daily across the borders that help keep power stations running, that help keep trains running, that help keep British manufacturing running - are those parts going to be able to pass pretty frictionlessly over the border?" Maier said.
"Or are we going to be in a situation where we have supply chains that are struggling to deliver to us and where we are struggling to export from the UK?" (Reporting by Guy Faulconbridge; editing by Michael Holden)