Spy agencies in Britain will risk having their methods revealed if Donald Trump stays true to his word and forces through the release of an explosive FBI memo.

President Trump is expected to authorise the publication of the classified document, which would have to be agreed by Congress, despite the FBI raising 'grave concerns' about the decision.

The memo is said to centre on a former MI6 agent and the ongoing investigation into Russian meddling and surveillance on the President during the 2016 presidential election campaign.

But it also includes sensitive information about how the likes of MI5, MI6 and the NCA operate, and an insider has said if the information is made public by Trump, the secret services would be less likely to come forward with their information in the future.

President Trump is expected to authorise the publication of the classified document despite the FBI raising 'grave concerns' about the decision

President Trump flew to West Virginia Thursday, where he will address House Republicans

The source told the Telegraph the document was based on the ex-spy Christopher Steele and his dossier against Trump.

'British intelligence is likely to be rankled because they view Christopher Steele as one of their own,' the insider said.

A number of experts on both sides of the Atlantic have said the release would hurt UK-US relations, but Trump appears set on giving it the go ahead.

A senior administration official said Trump hoped the memo would be published 'probably tomorrow'.

The staffer said Trump was 'OK with it' being released and doubted there would be any redactions.

Trump reviewed the memo and the White House made the decision, despite agency pleas, that it 'doesn't give away too much in terms of classification'.

California Rep. Adam Schiff (right) sent a letter to House intelligence committee chairman Devin Nunes (left) late Wednesday that charged the memo containing classified information about supposed surveillance of the Trump campaign was 'altered' by Republicans

Adam Schiff, ranking Democrat on the House Intelligence Committee, wants the memo to remain under lock and key amid concerns over its release.

But the Republicans want it released with critics claiming it will take the heat off the political party as the Russian probe deepens.

Writing in the Washington Post, talking about Britain, Schiff said: 'They will be far more reluctant to share their secrets with us in the future.

'Moreover, sources of information that the agencies rely upon may dry up, since they can no longer count on secrecy when the political winds are blowing.

'This is a grave cost for short-term political gain.'

Schiff wrote to House intelligence committee chairman Devin Nunes regarding the issue - saying the Republicans had altered the memo.

A spokesman for Intelligence chairman Rep. Devin Nunes of California said Democrats were 'complaining about minor edits to the memo, including grammatical fixes and two edits requested by the FBI and by the minority themselves.