London law firm Curtis & Co has tightened up security to avoid information leaks following a newspaper report relating to its former managing partner, Stephen Curtis, who died in a helicopter crash in Dorset in March.

The Financial Times claimed that Mr Curtis - who acted for some high-profile Russian tycoons - approached the UK intelligence services before his death offering to provide information about illicit Russian business activities in the UK.

It said the authorities were suspicious of the circumstances surrounding the crash.

The newspaper referred to an attendance note from a meeting held at the firm before Mr Curtis's death, which allegedly showed he had helped create a 'financial structure involving a web of shell companies' for a Russian oil company.

Curtis & Co consultant Robert Sprawson said: 'The picture painted in the press is not the man we knew.

Mr Curtis was an open man.

It has been suggested that he was somehow underhand or that he employed subterfuge, but he was not like that.

He was a man of complete integrity.'

He added: 'We are tightening security processes to make certain no other documents are obtained from the firm.

We are being very, very careful about our paperwork.

The last thing we want is for confidential information to leak.'

On the crash, Mr Sprawson said: 'Insofar as people do think there was foul play, these suggestions were fanned up by the press.

I have a completely open mind on the cause...

I will wait for the results of the official [air accident branch] investigation.'

Jeremy Fleming