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One in five of the top 350 UK companies have been warned about the lack of gender diversity at senior levels.

A government-backed review has written to 63 firms asking them how they plan to improve gender balance.

The Hampton-Alexander Review wants a third of board-level and leadership positions at the UK's top firms to be filled by women by the end of the year.

Figures show many are nowhere near the target - although the 100 top companies have passed one of the benchmarks.

The review was set up in 2016 by the former chair of RBS Sir Philip Hampton and the late CBI president Dame Helen Alexander.

It monitors the progress of women in leadership across FTSE 350 firms - the companies which make up the FTSE 100 and FTSE 250 stock market indexes.

Earlier this month, figures showed many companies had made good progress towards the review's target with women now holding one in three board positions at FTSE 100 firms.

But the review has singled out 24 FTSE 250 companies which only have one woman on their board and 35 FTSE 350 firms which have all-male executive committees.

Another four FTSE 250 companies failed to meet both targets - they had an all-male executive committee and only one woman on the board.

Of the firms warned that they had all-male executive committees, four of them were FTSE 100 firms: Ashtead Group Plc; Fresnillo Plc; Melrose Industries Plc; and Spirax-Sarco Engineering Plc.

'Diversity for long-term success'

The review worked alongside the Investment Association which represents major pension funds managing £8tn.

"Diversity results in better decision-making and plays an essential role in a company's long-term success," said Chris Cummings, CEO of the Investment Association.

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The review's chief executive, Denise Wilson, said: "Despite moving in the right direction, too many companies are not making progress quickly enough.

"If every FTSE 350 company achieved at least 33% women's representation on their board and in senior leadership by the end of 2020, this would be a major milestone.

"Increasing the representation of women in leadership and ensuring diversity in the widest sense, is a fundamental foundation to building a truly inclusive culture - where everyone can thrive."

How did we get here?

Since 2011, two government-backed reviews have focused on boosting women's representation on the boards of UK-listed companies.

The Davies Review, which ran from 2011 to 2015, increased the proportion of women serving on FTSE 100 boards from 12% to 25%.

For FTSE 250 firms, it increased the proportion of women serving on boards from 9% to 22%.

At the end of that review, Lord Davies recommended a new target of 33% of women board members at FTSE 350 firms by 2020.

Its successor, the Hampton-Alexander Review, adopted this target - it wants FTSE 350 companies to meet a 33% target by the end of 2020. Unlike countries including France which have introduced quotas, it is voluntary.