The FTSE 100 Index reached its highest point since June 2015, up more than 1% to 6,986.74, as investors were handed greater clarity over when Britain will exit the EU.
However, the pound was waning on the currency markets, dropping to a three-year low against the euro amid concerns that Mrs May will opt for a “hard Brexit” and leave the single market.
The announcement that Britain would begin the formal process of leaving the EU by triggering Article 50 in March 2017 also caused sterling to hit a three-month low against the US dollar.
However, the pound momentarily pared losses after output in Britain’s manufacturing sector reached its highest level for more than two years as the industry continued to bounce back from a post-Brexit vote slump.
The closely watched Markit/CIPS UK Manufacturing purchasing managers’ index (PMI) said output hit 55.4 last month, up from 53.4 in August, and above economist expectations of 52.1.
Sterling was down 0.7% against the dollar at 1.287 US dollars and off 0.7% against the euro at 1.145 Euros.