Shares in Standard Life and Aberdeen Asset Management jumped in early trade after the two fund managers confirmed they had agreed a merger.
The deal will create a company overseeing assets worth £660bn.
Standard Life shares rose 7%, while FTSE 250 member Aberdeen Asset Management climbed 5.4%.
Overall, however, the market was lower, with the benchmark FTSE 100 index down 26.88 points at 7,347.38 shortly after trading began.
Advertising group WPP fell a further 2%, adding to the 8% decline seen on Friday when it gave a downbeat forecast for the coming year.
On the currency markets, the pound fell 0.2% against the US dollar to $1.2271 and was also 0.2% lower against the euro at 1.1549 euros.