"We now expect the United Kingdom's net general government debt as a percentage of gross domestic product to continue to rise in 2015, before declining again," S&P said.
S&P noted "future employment or growth shocks could pressure government finances further."
S&P said the government's focus on fiscal discipline would hamper economic growth.
"We continue to believe the government's efforts over the next few years to engineer the planned correction in the U.K.'s fiscal accounts will likely drag on economic growth, although we note that the expected pace of consolidation is to ease in the short term," S&P said in a statement.
"We could lower the ratings on the U.K. within the next two years if fiscal performance weakens beyond our current expectations. We believe this could occur in particular as a result of a delayed and uneven economic recovery, or a weakening of political commitment to consolidation," S&P said.
"We expect economic growth to rise slowly in the medium term, with net general government debt as a percentage of GDP continuing to rise in 2015, instead of stabilizing in 2014 as previously expected."