LONDON (Reuters) - Barclays cut the overall pay of its chief executive Jes Staley last year, while an ongoing investigation into his efforts to unmask a whistleblower at the British bank could yet lead to him losing some or all of his 2016 bonus.

Staley's pay of 3.88 million pounds ($5.39 million) in 2017 was 8.5 percent less than he received the previous year, Barclays said on Thursday after reporting results.

The former JPMorgan (>> JP Morgan Chase & Company) investment banker, who took charge of the lender in December 2015, comes from a culture where bonuses are the barometer of performance.

Staley's payout is the smallest of the 2017 packages revealed by the top British banks so far, with state-backed Royal Bank of Scotland still left to report.

While he earned 2.35 million pounds in fixed base pay last year, Staley's bonus of 1.07 million pounds was down from 1.32 million in 2016 and less than half of the maximum available.

Crawford Gillies, chairman of Barclays' remuneration committee, said it would keep Staley's 2016 bonus under review pending the outcome of the Financial Conduct Authority (FCA) probe into his handling of the whistleblowing incident.

Barclays said Staley was awarded 1.69 million shares with a face value of almost 3.3 million pounds as at June 23, as part of the bank's 2017-2019 long term incentive plan.

The amount he actually receives is dependent on achievement of several performance objectives, including increasing shareholder returns and trimming costs.

Lloyds Banking Group boss Antonio Horta-Osorio and HSBC's departing chief executive Stuart Gulliver secured higher pay packages, despite missing analyst estimates on profits.

Barclays pledged to boost its dividend on Thursday despite a lackluster performance at its investment bank and below-forecast pretax profits hit by restructuring costs and fresh charges for historic misconduct and compliance failures.

The total Barclays group bonus pool shrank again by 2 percent to 1.5 billion pounds and is down by 57 percent in seven years.

(Reporting by Sinead Cruise and Lawrence White; editing by Alexander Smith)

By Sinead Cruise and Lawrence White