Bridge tolls became the first big bargaining chip in the Holyrood hustings yesterday when Jack McConnell pledged to scrap the charge for using the Tay Bridge and remove it for those car-sharing as they crossed the Forth.

The Liberal Democrats immediately let it be known that their manifesto would include an identical pledge, and yet both executive parties fought hard to retain polls during a parliamentary debate last month which saw six Labour MSPs and two LibDems rebel.

As Mr McConnell announced his intention to abolish the Tay Bridge toll, the SNP called the tolls gambit "a desperate and cynical attempt by Labour to try to buy back favour", pointing out Labour had voted against axing the charges in last month's debate. The Forth toll will remain, fixed at £1, but only for single-occupancy vehicles.

Both the SNP and the Tories have already promised to end the tolls.

Angus Robertson, the party's campaign director added: "They voted against tolls abolition only a few weeks ago, but it will backfire. People will say to themselves, if this is what happens when the SNP go ahead in the polls, let's keep voting SNP."

Tricia Marwick, SNP candidate for Central Fife, said: "This latest U-turn lacks all credibility. Unlike Labour the SNP has a clear and consistent policy to abolish all tolls on the Forth and Tay bridges - what Labour are proposing for the Forth is a dog's breakfast and would leave it as the only bridge in Scotland with a toll."

A Conservative spokesman said: "It's nice to see that, again, where we lead, the others follow. However, this does not go as far as our proposals and many people will still end up paying. It is a desperate attempt from Labour to shore up their crumbling support but it is far too little, far, far too late."

Mr McConnell made his announcement on the Tay Bridge toll on the campaign trail in Dundee, a city where one of the two seats has already been lost to the SNP and the other is under threat.

Mr McConnell insisted it would be "irresponsible" to completely abolish charges on the Forth Road Bridge while detailed plans are being drawn up for a new crossing. The Scottish Executive has committed to build a £1bn replacement for the Forth Road Bridge, which may have to close to heavy lorries by 2013 because of corrosion in the metal cables.

The First Minister said: "I am still concerned that removing tolls completely on the Forth Bridge could lead to increased traffic congestion, and ending the revenue stream from tolls before detailed costs and proposals are prepared for a new crossing of the Forth is simply irresponsible."

A spokesman for the National Alliance Against Tolls welcomed the promise to remove tolls on the Tay Bridge, but said they were dismayed Labour would still keep some charges on the Forth, which they saw as a prelude to congestion charging and eventually road pricing.

Against the backdrop of abolition of tolls over the Erskine Bridge over the Clyde and the Skye Bridge, the Tay and Forth charges have become increasingly controversial, but with the latter crossing facing structural problems and the approaches to Edinburgh facing serious congestion, the executive held firm against abolition.

The next big bargaining chip at the electoral table is likely to be the long-awaited announcement about a contract for two aircraft carriers. Govan could have a major role, while Rosyth would be the likely yard for final assembly.

There was much speculation yesterday that an announcement could be made during a visit by Tony Blair to Scotland on Monday - the last day on which major government announcements can be made before the formal start of the election campaign.