The UK's left-leaning newspaper, The Guardian, has scored a hit with readers following its format change from broadsheet to Berliner.

Unofficial figures say sales increased by a spectacular 40% on the first day of the new size. The newspaper's average headline circulation for August was 341,689, suggesting that on Monday it could have sold more than 450,000 copies.

Owner, the Guardian Media Group has invested more than $80 million ($145m; €118m) in the revamp, including the purchase of three new German printing presses and an estimated £2m ad campaign to support the relaunch.

Says Guardian Newspapers ceo Carolyn McCall: "We invested a huge amount in encouraging readers to reappraise the paper. This has exceeded our highest expectations. We are delighted."

Although the dramatic gains of the first day are not expected to be sustained long term, the paper must increase circulation significantly to recoup the investment.

Data sourced from Brand Republic (UK); additional content by WARC staff