The issue of the Dounreay nuclear plant has been hovering over ministers like a radioactive cloud for months.

Now, in one of the most embarrassing U-turns of this government, they have announced it is to be shut down. But it is doubtful that will bring an end to the controversies that have surrounded the plant since it was built in the 1950s.

Successive governments have had to deal with scare stories and genuine problems with the plant - but Tony Blair has had more than his fair share in a short time.

Just over a month ago he was forced to tell MPs in the Commons that there had been no government cover-up over plans to ship 5kg of uranium and spent nuclear fuel from Georgia, in the former Soviet Union, for disposal at the plant.

But details of the shipment had been kept secret and were only known about once the waste was on its way to Scotland.

It only made matters worse when MPs discovered that both Russia and France had refused to take the consignment, which it was claimed could be used to build a small nuclear bomb.

The prime minister's insistence that the decision had only been taken in a bid to stop the material falling into the wrong hands did little to calm the row.

Counter-attack on the SNP

Tony Blair insists Alex Salmond's assumptions are based on a misinterpretation

Only weeks later, Mr Blair was back on the rack over claims that enough material had gone missing from the plant over the years to make 12 nuclear bombs.

He insisted the information was wrong, and based on a misinterpretation of 30-year-old figures. But he also turned on the Scottish National Party, accusing it of spreading irresponsible scare stories.

He snapped at SNP leader Alex Salmond: "To alarm the public in this way is irresponsible, extreme and entirely typical."

And he bluntly rejected Mr Salmond's demand for an end to future shipments of material to the plant.

Scottish rivalry

The strength of the prime minister's attack was calculated to damage the SNP, which is proving to be a real threat to Labour north of the border.

Recent opinion polls suggest that the SNP will win far more seats in the new Scottish Parliament than the government ever predicted and could even beat Labour into second place.

The sight of a member of the SNP becoming Scotland's first prime minister - rather than one of Mr Blair's candidates - terrifies Labour and Mr Blair is determined to fight back.

At the same time that Mr Blair was lambasting the SNP, energy minister John Battle was insisting Dounreay was entirely safe.

Tactic backfires

But the tactic has dramatically backfired with the surprise announcement that the plant is to be shut down and will receive no more consignments of waste.

Both Mr Blair and Mr Battle have been left with egg on their faces while Mr Salmond has been handed a significant propaganda tool to use against Labour in the future.

The sudden decision, slipped out in a written Commons answer and at a news conference rather than a full statement to MPs, has also added to the impression that government is ready to cave in on any issue which sparks a big enough row.