Blue chip shares jumped in afternoon trading as the pound weakened after Prime Minister Theresa May said the UK will trigger Brexit negotiations by the end of March.

The pound fell 0.72% against the dollar to $1.2874, close to a 31-year low against the greenback. Sterling dropped 0.74% versus the euro to €1.1457 after May revealed the timing of triggering Article 50 of the Lisbon Treaty, which means the UK will leave the European Union by the summer of 2019.

The FTSE 100 Index lifted 68.8 points to 6968.2, as top flight shares hit their highest level this year. The FTSE 250 Index soared by 287.5 points to 18159.

IG chief market analyst Chris Beauchamp said: "A slump in sterling made life easier for UK markets, with price action so far seeing high-quality international firms receiving a flood of buyers."

In the second tier shares in UK-listed Henderson Group surged after it announced a merger with US firm Janus Capital, creating an investment titan with more than $320bn (£247bn) in assets under management.

The two firms said their "merger of equals" will see the combined entity renamed Janus Henderson Global Investors and hold a combined market capitalisation of $6bn. Henderson jumped 39p to 271p.

In afternoon trading the biggest risers in the FTSE 100 Index were London Stock Exchange (+80p to 2880p), Travis Perkins (+44p to 1588p), Smiths Group (+36p to 1500p), Bunzl (+52p to 2332p) and InterContinental Hotels (+72p to 3252p).

The biggest fallers in the FTSE 100 Index were Marks & Spencer (-6.5p to 324.6p), Legal & General (-3.1p to 215.6p), British Land Company (-8.5p to 624p), BT Group (-5.2p to 384p) and Barclays (-2p to 165.8p).

In afternoon trading the biggest risers in the FTSE 250 Index were Henderson Group (+39p to 271p), Evraz (+16.6p to 177.6p), Countryside Properties (+14.4p to 257.2p), PayPoint (+58p to 1090p) and OneSavings Bank (+13.9 to 267.6p).

The biggest fallers in the FTSE 250 Index were Great Portland Estates (-6.5p to 626.5p), Ocado Group (-2.6p to 262p), B&M (-2.1p to 253p), N Brown (-1.4p to 188.5p) and Inmarsat (-5p to 700p).