Barack Obama today reassured Prime Minister Gordon Brown over his plummeting popularity.

Speaking outside 10 Downing Street after two hours of talks this morning, Senator Obama said he had no advice for Mr Brown but added: "You're always more popular before you're actually in charge. Once you're responsible then you're going to make some people unhappy."

Mr Obama said he and Mr Brown had talked about strengthening the relationship between the US and Britain.

The Democrats' candidate said he has already committed to increasing the number of troops in Afghanistan if he is elected. But he hinted that Britain would also be expected to offer support when he added: "We'd like that burden shared."

Earlier he had a breakfast meeting with Tony Blair and will later meet David Cameron.

Blair arrived at Obama's five-star hotel near Marble Arch at 7.30am in his capacity as a Middle East envoy.

The former PM's spokesperson said: "They exchanged views on the situation in the Middle East and its wider implications, in light of Senator Obama's visits this week and Mr Blair's continuing efforts to promote peace and prosperity," he said.

"The two also discussed Tony Blair's work on climate change. The recent report of Mr Blair's 'Breaking The Climate Deadlock' initiative has set out the framework for a new global deal for a low carbon future.

"It identifies the actions and questions that need to be resolved by political and business leaders over the next 18 months to achieve a successful outcome to the UN climate change negotiations in Copenhagen in December 2009."

The meeting at the Churchill hotel in London's Portman Square lasted for just over an hour.

Mr Blair has previously met Senator Obama in both London and Washington, and also met his Republican opponent SenatorJohn McCain when he was in London earlier this year.

The UK visit is the final leg of a tour designed to persuade American voters of his foreign policy credentials.

He has used the trip, which has taken in Iraq, Afghanistan, Israel and the Palestinian Territories, to promise repairs to cross-Atlantic relations if he defeats Republican John McCain in November's contest to replace George Bush in the White House.

America was now too often seen in Europe as "part of what has gone wrong in our world, rather than a force to help make it right", he said.

"If we're honest...we know that sometimes, on both sides of the Atlantic, we have drifted apart and forgotten our shared destiny."

A relaxed looking Mr Obama landed in the UK last night in a plane emblazoned with the words "Change we can believe in" and was met by the US ambassador and his wife, Robert and Maria Tuttle, before being taken to the five-star Hyatt Regency London - The Churchill.

He is due to fly back to the US tonight to resume battle with Mr McCain, who visited No 10 for talks in March, while his rival was still battling Hillary Clinton