Tesco leads slide on FTSE 100 index
Shares in Tesco led the declines on the FTSE 100, giving up some of Friday's gains when the stock surged in the wake of news of its planned tie-up with food wholesaler Booker.
The supermarket's stock fell by more than 4% to close at 197.8p.
Over the weekend, press reports suggested the deal faced a lengthy investigation from competition regulators.
Shares in Booker also fell, with the FTSE 250 company down 3.6%.
Overall, the FTSE 100 ended down by 66 points at 7,118.48. Most major stock markets around the world fell after President Donald Trump's decision to impose a travel ban on seven Muslim-majority countries.
Among the gainers in the UK, Vodafone rose after the mobile phone company said that its Indian unit was in merger talks.
A combination of the two companies would create India's biggest telecoms firm. Shares rose 1.3%.
On the FTSE 250, shares in WS Atkins jumped 6% following a report in the Times that the engineering company had been approached by US company CH2M over a possible merger.
Flybe shares dipped 1.1% after the airline said it had seen a "slow start" to trading since the start of the new year, blaming "uncertain consumer confidence and poor weather".
That was despite the carrier reporting "solid" trading for the final three months of 2016, with revenues boosted by extra capacity. Flybe said passenger revenues rose 13.5% in the quarter, compared with 5.7% in the previous six months.