Britain's most powerful shareholder group has said it has concerns about executive pay levels at Barclays Plc, adding to growing criticism over a 17 million pound award for Chief Executive Bob Diamond.

IVIS, the shareholder advisory service of the Association of British Insurers (ABI) - whose roughly 440 members own about 20 percent of the FTSE All-Share index - said it had issued an amber top warning on Barclays' remuneration, which indicates it has some concerns. A red top warning signals its most serious criticism about an issue.

The ABI said its concerns were about executive pay. It had already criticised Barclays after its 2011 results two months ago, saying the bank had not gone far enough in cutting staff pay to improve rewards for investors.

It is not the signal of the change required in order to improve the investment case, Robert Talbut, chairman of the ABI's investment committee, said at the time.

Since then, Barclays said Diamond took home about 17 million pounds in salary, bonus and share awards for last year, once again making him one of Europe's highest paid bankers.

Pirc, another shareholder advisory service, said investors should reject Barclays' pay plan at the bank's annual shareholder meeting on April 27.

Once again, disclosure is nebulous and the remuneration structure remains complex, Pirc said last week, adding that executive awards should be clawed back after an insurance mis-selling scandal involving all UK banks.

In view of the fact that Barclays' shares are trading far below net asset value, we cannot think of any circumstances in which a Chief Executive who was part of a team when the bank got into that predicament should be receiving any bonus at all; indeed the board should also be considering clawbacks itself, Pirc said.

Some of Britain's biggest institutional investors are considering voting against the remuneration report, newspaper reports have said.

Barclays said on Tuesday it had extensive engagement with shareholders, including the ABI and its members, over recent weeks.

We note the concerns and questions that they have raised in their report and look forward to continuing to work with them on those, a spokesman said.

(Reporting by Steve Slater; Editing by Richard Chang)