* ICM poll: May's lead down to 12 points from 14
* Survation poll: May's lead down to 6 points from 9
* Other recent polls show a decline in May's rating
* Sterling rises on ICM poll
LONDON, May 30 (Reuters) - British Prime Minister Theresa May's ability to deliver a landslide majority in the June 8 election was in doubt on Tuesday after two more opinion polls showed the vote could be much tighter than previously thought.
A total of seven polls carried out since the May 22 Manchester attack have shown May's lead over the opposition Labour Party narrowing, with some suggesting she might not win the landslide predicted just a month ago.
A poll conducted by Survation for ITV's Good Morning Britain programme showed May's lead had dropped to 6 percentage points from 9 points a week ago and 18 points two weeks ago.
An ICM poll for the Guardian showed May with a 12-point lead - enough for a big majority of around 100 but down two points from last week and a far cry from the record 22-point lead earlier this month.
"Three weeks ago this was the easiest election to call in history," Martin Boon, ICM's director, told Reuters.
"But since the manifestos were launched, there's been a rapid tumbling in the gap between the Conservatives and Labour," he said, referring to the pre-election pledges of the main parties.
May's poll lead started to contract sharply after she set out plans on May 18 to make some elderly people pay a greater share of their care costs, a proposal dubbed the "dementia tax" by opponents.
As her lead shrank, May was forced to backtrack on the policy at an appearance before the media on Monday at which she appeared flustered and irritated when taking questions from reporters.
The polls so far have been hard to decipher with both the unpopular social care pledge and the May 22 suicide attack in Manchester influencing voter intentions.
Sterling fell on Friday on concerns May's lead was under pressure but climbed to the day's high against the dollar on Tuesday after the ICM poll.
May called the snap election in a bid to strengthen her hand in negotiations on Britain's exit from the European Union, to win more time to deal with the impact of the divorce and to strengthen her grip on the Conservative Party.
But unless she handsomely beats the 12-seat majority her predecessor David Cameron won in 2015, her electoral gamble will have failed, and her authority could be undermined just as she enters formal Brexit negotiations.
Labour commands a big lead among young Britons, but recent votes show they are the least likely to go to the polling stations.
"We shouldn't write off the thought that young people will turn out for this election (in large numbers) - but I think it's unlikely," Boon said.
Both the Survation and ICM polls were conducted in the aftermath of a suicide bombing which killed 22 people in Manchester last Monday.
It is still unclear how much Britain knew about suicide bomber Salman Abedi before he carried out the deadliest militant attack on British soil for 12 years. May was interior minister from 2010 to 2016.
The ICM poll showed 53 percent of Britons thought May handled the situation well. Only 17 percent disagreed.
The Survation poll found just over half of the 1,009 respondents thought May would make the best prime minister, whilst support for Labour's Jeremy Corbyn stood at just 30 percent, albeit higher than in previous surveys.
(Reporting by Costas Pitas; Editing by Hugh Lawson and Guy Faulconbridge)