Mr Milligan was the Parliamentary private secretary to Jonathan Aitken, the Minister of State for defence procurement. But the Prime Minister's office said Mr Milligan had no access to classified documents.
'There is no security angle to Stephen Milligan's death. He had no access to classified information, no office at the Ministry of Defence. He worked only on the Parliamentary and political side,' a source said.
It is almost certain that Mr Milligan would have attended meetings with other ministers at the MoD where classified items were discussed, such as defence contracts, although he may not have been involved with top secret material.
His death raised questions about the vetting procedures for MPs. They are not routinely positively vetted, with MI5 checks on friends and relatives. But there was no pressure for that to change.
'Positive vetting would not have discovered anything,' said one Labour MP who, as a former civil servant, went through positive vetting. 'MI5 asked my friends whether I blabbed when I got drunk. They said 'no' and he went away satisfied.'
The selection procedure is likely to continue on the 'Old boys' network', according to Tory MPs who have been through it. 'Constituencies sometimes ask Central Office whether anything is known,' one Tory MP said.
The whips are said to keep tabs on their MPs to know in advance about likely scandals. 'We knew about David Mellor before the news of his affair leaked out,' said a whip. 'But there's not much you can do.'
The whips believe there was little likelihood of their finding out about Mr Milligan's apparent sexual perversion before his death. 'If MPs won't tell their wives or girlfriends, it's hardly likely they will tell the whips,' the whip added.