There are summer storm clouds on the horizon for Labour as relations between the government and key public sector trade unions continue to worsen.

With council workers already involved in a dispute, postal workers to vote on industrial action and the Fire Brigades Union set to follow suit, the prospect of a series of national strikes is a growing headache for ministers and Labour Party officials alike.

As trade unionists vote out moderate Blair allies such as Sir Ken Jackson at Amicus and union bosses cut donations to party coffers by £1.7 million, Conservatives say Labour is undoing the reforms of the Thatcher era.

Following the latest news that airport baggage handlers and check-in staff could turn to industrial action in their pay dispute, shadow transport secretary Tim Collins said that "one of Labour's biggest paymasters, the GMB, are planning to ruin even more holidays in pursuit of unnecessary strike action".

"Government ministers should use their much publicised influence with the trade unions to get this strike called off," he said.

But with Labour £6 million in the red, the financial need for big cash donations from the unions has prompted debate over the best way to raise party funds.

While Labour has already raised membership fees and senior ministers have begun a public debate over introducing state funding for political parties, Charles Clarke has now urged the unions to agree a three year funding deal to put financial relations on "a proper footing".

The Labour chairman admitted that the recent funding cuts amounted to "a significant and unexpected reduction in funding for the party at a difficult financial time".

Writing in Tribune magazine, he added: "The affiliated trade unions have to agree a stable funding regime for the party. This regime could be at a high, low or steady level, but it must be stable for, say, a three-year period."

The call has prompted a mixed response from the unions, but along with problems managing its own finances Millbank is also facing newspaper headlines predicting a "summer of discontent".

The disputes have different causes, from pay claims to government plans to reform key public services.

Leaders of the GMB, Unison and TGWU unions are continuing talks with council representatives over a deadlocked pay dispute that could see a second day of strikes around the country.

The union leaders, which represent 1.2 million council workers, are demanding a bigger pay rise than the current offer of three per cent, but local authority leaders have described their claims as "pie in the sky".

Without a deal, a second day of strikes in England, Wales and Northern Ireland will go ahead on August 14.

Elsewhere, teaching unions have criticised a big shake-up in performance related pay being put forward by the education secretary.

Estelle Morris wants to establish a stronger link between performance and bonuses, worth up to £4356 to senior teachers, based on the judgment of school heads. The reforms would be phased in over the course of a three-year pay deal.

Teaching unions had wanted the cash to go on above inflation increases in basic pay. David Hart, general secretary of the National Association of Head Teachers, warned the plans amounted to "a recipe for a rash of industrial disputes at school level".

With the teachers' pay review body to announce its verdict on the reforms early in January, education ministers face more tough negotiations with unions.

Meanwhile, on Thursday the Communication Workers' Union announced a ballot of its 180,000 members over plans to privatise the jobs of 7000 Post Office cleaning and maintenance workers.

The CWU will start balloting postal workers on August 27 and announce the result on September 12, when a positive vote could see the first national strike since 1971.

Also on September 12, the Fire Brigades Union is set to announce a ballot over a pay claim of £30,000 a year.

The two planned announcements leave Tony Blair facing a difficult task when he addresses the Trades Union Congress on September 10.

The Local Government Association, biggest fire service employer, has accused the FBU of trying to "hold a gun to the heads of their employers".

The LGA warned ministers that the union's pay claim would cost an additional £440 million - plus corresponding national insurance and pension increases

The local government body says that ministers should attempt to break the current impasse in order to ensure a compromise deal can be done on pay and conditions.

"It seems that the FBU are putting their own pay before the shared goals of investing in better public services urgently needed to protect people's safety and quality of life," said the LGA's John Ransford.

Its a sentiment that seems to be shared by the public.

A recent Guardian/ICM opinion poll found public concern that Gordon Brown's planned increases in public spending would be swallowed up in inflation-busting pay deals.

The survey found 33 per cent think there is a "big danger" of it happening, with 45 per cent believing there is a "slight danger" and only 13 per cent agreeing that there is "no danger". Among Labour voters there was only slightly less anxiety.

However, the same poll also found backing for the strikes held by council, rail and Tube workers.

Among the general public, 59 per cent gave their backing to strikes, with a slightly higher figure of Labour voters, 61 per cent, doing the same. There was opposition from only 29 per cent of voters.

It adds up to a difficult balancing act for ministers as they deal with complex and related problems of wage demands, government spending, public sector reforms and party finances.