Nationalist councillors have been warned not to be rushed into coalition deals simply to secure extra pay for senior posts, as party headquarters seeks to keep control of post-election horse-trading.

However, they have also been told they will have to move quickly if they are to form new cross-party administrations in many of Scotland's 32 councils.

The Scottish National Party's national secretary has sent candidates a letter, leaked to The Herald, offering guidance on how to handle the uncertain aftermath of the council election, which will mark a dramatic change from the previous party balance across much of the country.

Research shows part of that change is set to be accompanied by a fall in the number of women councillors from all parties.

The letter warns SNP candidates they should not form "formal coalitions" with Conservatives, as that would be barred by party policy dating back to the anti-poll tax campaign. However, with few councils having majority control, the SNP is likely to be forced into softening that stance on the Tories, and informal alliances are expected.

The memo states: "Our opponents will be seeking to make deals as quickly as possible during and after the counts on 3/4 May, and may offer individual councillors particular positions in return for support or agreements on individual policies. They may target individuals, seek to split the SNP group and exert considerable pressure".

National secretary Duncan Ross suggests: "It may be very tempting to achieve a local or ward- specific goal in return for a committee chairmanship, for example. It is vital that you do not enter into any such agreements without first speaking to your colleagues". He also says all coalition deals have to be approved by the SNP's national executive.

The guidance reflects concern among all the parties at national level that the unpredictable outcome of the new voting system in today's council election will leave many inexperienced councillors in positions of power. While Labour is expected to lose at least 200 councillors simply because the system is more proportional, many with experience have stood down, taking a pay-off of up to £20,000 to do so.

Councillors elected today will get a sharply increased payment for their work, of a basic £15,000 per year with enhanced payments for those taking on committee and leadership responsibilities.

There are 1222 councillors being elected today, for 353 new wards, each having either three or four councillors. There has been a sharp drop in the number of candidates from around 4200 at the 2003 election to nearly 2600 today - largely because parties that stand too many candidates may find their support splitting between them, risking defeat for all.

An academic study of the candidates shows that the new voting system, intended to make councils reflect the Scottish population better, has had the opposite effect in reducing the number of women candidates for the main parties.

Professor David Denver of Lancaster University has studied today's candidate lists and, after a generation of steadily rising female candidacies, he says that has fallen markedly, from 28% in 2003 to 23% today. Among those elected four years ago, 22% were women.

The percentage of women candidates for Labour has fallen from 26 to 20%, SNP from 25 to 22%, LibDem from 37 to 31% and Conser-vative from 32 to 26%.

As an explanation, Professor Denver suggested: "In the battle for fewer places available, because they're cutting the numbers anyway, it's the women that have lost out.

"Some people thought the Single Transferable Vote system would allow for balanced tickets, including women or ethnic minority candidates, but plainly, that's not happening".

Professor Denver has also calculated that of the 353 wards, Conservatives are standing more than one candidate in only 43, Labour in 180, LibDem in 57 and the SNP in 96. Labour is targeting three candidates at 37 heartland wards, but few other parties are taking that risk. There are 163 wards with four councillors, and 190 have three.