Diamond, who was unavailable for comment, is also expected to continue in his role at WS, but will now have management oversight of WS, Golin/Harris International and The MWW Group.
The firms' combined revenues for 2001 - the last year records were available - were £250m.
While each firm will remain a separate operating unit, the senior management will report to Diamond, who in turn reports to recently appointed IPG chief exec David Bell.
Weber - who sold his PR firm, then called Weber Group, to IPG in 1996 - acts as CEO of the Advanced Marketing Services (AMS) group, which contains all of the PR holdings now overseen by Diamond.
Weber said he is still head of AMS, but in a recent Securities and Exchange Commission filing, IPG hinted at dissolving AMS after it sells market data firm NFO Group.
WS joint UK CEO David Brain remained tight-lipped on the senior staff shuffle, but insisted the changes will not affect the UK business.