Angus Robertson was given a “secret” wage top-up of £33,000 while SNP Westminster leader, it has been revealed.

The veteran politician received the pay bump after the SNP’s 2015 landslide general election victory in Scotland, when it won 56 out of 59 seats and became the UK’s third-largest party.

Robertson led the party in the House of Commons for a decade until 2017 and is now the Scottish Government’s constitution secretary.

Scottish Labour condemned a “festering culture of secrecy” after it was revealed the £33,000 salary boost was not made public.

It is the latest financial revelation to cause controversy as the SNP faces a Police Scotland probe into its funding that has been detectives arrest former chief executive and Nicola Sturgeon’s husband Peter Murrell and then national treasurer Colin Beattie.

Both men were questioned for nearly 12 hours before being released without charge pending further investigation.

At Westminster, the leader of the opposition, who stands opposite the Prime Minister during PMQs, normally receives a t

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