UPDATE 3-Trump praises UK PM Johnson as 'very capable' ahead of election

* Trump says he doesn't want to get involved in UK election

* U.S. President affirms support for Johnson and Brexit

* Trump says UK health system not involved in trade talks (Adds Johnson, Corbyn response)

By Steve Holland and William James

LONDON, Dec 3 (Reuters) - U.S. President Donald Trump said on Tuesday he thought Prime Minister Boris Johnson was very capable and would do a good job, moments after saying he did not want to get involved in the British election campaign.

The Dec. 12 election will decide the fate of Brexit and the world's fifth-largest economy: Johnson has promised to take Britain out of the EU on Jan. 31 while opposition Labour Party leader Jeremy Corbyn has promised another EU referendum.

Trump has featured heavily in the British election campaign, where his name is used by the Labour Party as a by-word for a capitalist system that Corbyn, a veteran socialist campaigner, has promised to tear up if he wins power.

In London to attend a meeting of NATO leaders, Trump said he did not want to complicate the election, but then added:

"I think Boris is very capable and I think he'll do a good job." He has previously cast New York-born Johnson as Britain's Trump and feted him as the best leader to deliver Brexit.

Johnson's Conservative Party are ahead in opinion polls, but it remains unclear whether they are far enough in front to form the majority government Johnson requires to take Britain out of the EU in January.

Trump is a divisive figure for some in Britain, and senior Conservatives are nervous his involvement could upset their campaign. Johnson said good relations with the U.S. were vital, when asked if Trump was an embarrassment.

Trump also reaffirmed his support for Brexit - the central objective of Johnson's election campaign - and denied that the U.S. was interested in gaining access to Britain's National Health Service (NHS) in a future trade deal - the main element of Corbyn's election campaign.

Corbyn wrote to Trump on Monday to ask him to revise U.S. negotiating objectives for a post-Brexit trade deal to ensure the NHS is not included.

Asked if he could work with Corbyn, Trump said: "I can work with anybody, I'm a very easy person to work with."

Trump was asked if the NHS could be on the table in post-Brexit trade talks and responded "No not at all. I have nothing to do with it. Never even thought about it, honestly."

Corbyn dismissed Trump's assurances and referred to a remark made by Trump in June, when he said everything was on the table in trade talks with Britain.

Labour say Johnson wants to sell parts of the publicly funded system off to U.S. companies. Johnson has repeatedly said that is not true.

Trump said: "We have absolutely nothing to do with it, and we wouldn't want to. If you handed it to us on a silver platter, we'd want nothing to do with it." (Reporting by Steve Holland, William James and Kylie MacLellan; editing by Alistair Smout/Guy Faulconbridge)