Blue chip shares regained their poise after global markets slumped earlier in the week amid the row over US President Donald Trump's links with the Kremlin.
The FTSE 100 Index lifted 28.7 points to 7465.1 in afternoon trading, after world markets fell in sessions following clashes over the firing of FBI director James Comey, which led to growing scepticism about Trump's ability to deliver tax and regulatory reform. The FTSE 250 Index rose 100 points to 19791.7.
IG chief market analyst Chris Beauchamp said: "Markets have continued their gradual recovery from the panic of Wednesday's session, with the FTSE 100 now less than 1% from its all-time high."
In London, Traders remained confident in the prospects of Just Eat despite an announcement by the Competition and Markets Authority (CMA) that it will probe the proposed £200m ($260m) takoever of online takeaway ordering rival hungryhouse amid concerns the deal could curb competition.
The CMA said it would launch an "in-depth" probe over fears that "the merger could lead to worse terms for restaurants using either company".
However, shares in the FTSE 250 firm lifted more than 2%, or 15p to 607p. Shares have jumped 18% since the middle of February.
The biggest afternoon risers in the FTSE 100 Index were Fresnillo (+37p to 1618p), ITV (+3.8p to 195.8p), Next (+80p to 4434p), Anglo American (+18.5p to 1085.5p) and Smurfit Kappa Group (+36p to 2135p).
The biggest afternoon fallers in the FTSE 100 Index were Hikma Pharmaceuticals (-80p to 1620p), Smiths Group (-63p to 1566p), Royal Mail (-10.1p to 423.5p), Johnson Matthey (-71p to 3009p) and Experian (-31p to 1629p).
The biggest afternoon risers in the FTSE 250 Index were Berendsen (+58p to 1104p), Senior (+11.2p to 233.4p), Petra Diamonds (+5p to 134.7p), Renishaw (+130p to 3517p) and Safestore Holdings (+15.6p to 439p).
The biggest afternoon fallers in the FTSE 250 Index were Hays (-7.5p to 165.8p), Thomas Cook (-4p to 91.3p), Euromoney Institutional Investor (-36p to 1180p), Ashmore Group (-10.3p to339.7p) and Pagegroup (-12.7p to474p).