LONDON, April 22 (Reuters) - The number of women holding top jobs at leading financial institutions has edged up to a record high, a new report has shown, suggesting diversity is becoming increasingly embedded despite the political backlash in the United States.

The annual Gender Balance Index by the London-based OMFIF think tank showed women now hold 19% of leading roles at central and commercial banks, sovereign and pension funds, up from 16% last year, helped by U.S. institutions like the Federal Reserve, where six of the regional bank heads are now women.

Based on data from 335 institutions, Wednesday's report showed, however, that the challenge of closing the gap with men remains significant.

The index's overall score of 44 out of 100 is still less than halfway to gender balance. The share of women holding broader senior roles inched up fractionally to 33% and it remains systematically difficult to land the top job, the research found.

"Even with the rate of improvement seen in recent years, it would still take 22 years for the GBI score to reach an average of 100," said Andrea Correa, the report's author, adding it could take even longer.

CENTRAL BANKS LEAD GAINS

Nevertheless, the improvement in North America - where the score has jumped to 82 - was reassuring given U.S. President Donald Trump's push to curb diversity, equity and inclusion programmes.

"It was a little bit of a surprise," Correa said. "Institutions have had to be more careful with their public messages (about equality), but they are still committed to it."

The biggest improvement came at central banks where a record 35 of the 185 in the index are now female-led. Sovereign funds, which manage a nation's wealth, also showed their biggest improvement in five years with the share of female CEOs up to 18%.

GLASS CEILING INTACT

Commercial banks saw little improvement though with still just seven women CEOs out of the 50 surveyed. The number heading pension funds slipped to 11 from 12.

Talent pipelines also remain thin: the overall share of women in executive C-suite roles stayed at 20% and almost half of the commercial banks in the index still have no women in their C-suite.

The report also introduced a "glass ceiling ratio" - the degree to which women's representation narrows at the top of institutions.

The index-wide figure - at 0.56 - showed they are represented in top roles at just over half the rate that would be predicted by their share of leadership positions overall.

Commercial banks have the lowest score of any institution type at 0.41, having both a higher-than-average representation of women in senior roles, but also lower-than-average share in the top job.

"There is still a ceiling for women," Correa said, adding that part of the reason was that reaching the top demands long working hours.

"That is often difficult for women because of child care and their role in households," she added, describing it as a "persistent promotion bottleneck".

(Reporting by Marc Jones; Editing by Emelia Sithole-Matarise)