Washington, Mar 28 (IANS): US President Joe Bidens long career in politics has been littered with gaffes, but his comment that Russian President Vladimir Putin "cannot remain in power" may be the most serious yet, The Times, UK, reported.

Biden's remark is certain to revive critics' claims that the American president, a self-confessed "gaffe machine", is suffering from serious cognitive decline.

The US President's comment in Warsaw, which drew a stinging response from Moscow over the weekend and overshadowed days of painstaking work to rally European allies against Russia, is not his first gaffe on the Ukraine crisis, The Times reported.

Biden, 79, later backtracked his remarks when a reporter asked, "Mr President, were you calling for regime change?" Biden yelled "No!" before getting into his motorcade.

Biden's insulting remarks about Putin could be the biggest misstep of his presidential career, the report said.

Earlier, about 40 Republicans demanded that Biden take a cognitive test and release the results to the public. The 79-year-old politician refused the test, and the President's medical team said that Biden was healthy, but had developed a "stiffer and less fluid" gait due to his advanced age, Russian media reported.