Mr Brown pledged to support the "fullest possible investigation"

The letter to Met temporary commander Nigel Mawer follows revelations that businessman David Abrahams made donations to the party through four proxy donors.

Dear Commander Mawer,

I wanted to write to you at the outset of your investigation to make it absolutely clear that you will not only have my co-operation in your enquiries but my support for the fullest possible investigation.

What happened in relation to these donations was unacceptable and it is in the public interest that any question of impropriety is answered.

To that end, I have asked all Labour Party staff, Officers of the Labour Party, Members of Parliament and Peers, to co-operate fully in providing relevant information. I will urge all former members of staff to follow suit. And all should be pro-active in providing information to you which would be relevant to your enquiries.

And of course I will remind people that it is also vital that you are able to proceed with your enquiries and produce your findings without any running commentary in the media.

I stand ready to assist your enquiry in any other way I can.

Yours sincerely,