London's mergers and acquisitions bankers fear a second multi-billion-pound merger in less than a week may fall victim to the financing drought for big deals.

Swiss drugmaker Roche is struggling to raise a £29 billion syndicated loan to finance its acquisition of US biotech group Genentech. Roche has been talking to banks in London since July but they refuse to underwrite any new loans before the year end.

That puts pressure on Roche to look at other funding options or drop plans to acquire the 44% of Genentech it does not own. It last week reiterated its commitment to do a deal it first announced more than four months ago.

The credit crisis has already led to the withdrawal of a record number of deals. Last week, market turmoil forced Anglo-Australian mining group BHP Billiton to pull its acquisition of Rio Tinto, which it was planning to fund with £36 billion of loans.

The collapse in deals is hitting banks revenues hard because they earn a large portion of their fees when a deal completes.

Deal activity is down by more than a third this year as banks have stopped lending to firms seeking deals.

Company bosses are also nervous of doing deals in volatile markets.