The late Polish President Lech Kaczynski's family does not want to delay Sunday's funeral despite a cloud of volcanic ash spreading across Europe that has forced the closure of many airports.

‘I wish to say that the family's will is that the date of the funeral should not be postponed under any circumstances,’ presidential aide Jacek Sasin said.

There has been speculation of a postponement to allow world leaders to fly safely to Poland to attend the funeral.

Krakow Airport, which would handle most arrivals, was shut today. along with most other Polish airports, due to the volcanic ash cloud spreading from Iceland

A number of foreign dignitaries are due to fly to Krakow, including President Mary McAleese, US President Barack Obama, Russian President Dmitri Medvedev and German Chancellor Angela Merkel.

Mr Kaczynski was among 96 people, mainly senior dignitaries, killed last Saturday when his military aircraft crashed when trying to land in western Russia in thick fog.

Earlier, Mr Sasin told Polish radio: 'I think it's a very serious alternative which must be taken into consideration.'

However, an official at presidential chancellery said delaying the funeral would be a 'last resort'.

Poland's aviation agency said that the country's airspace is closed until further notice.

The cloud spread over Poland late yesterday, causing authorities to shut down the north of the country's airspace.

Polish officials yesterday said that nearly 80 planes carrying world leaders were due to land at Krakow over a six-and-a-half hour window on Sunday morning ahead of the funeral.

The funeral is due to be held in the cathedral crypt of Krakow's Wawel castle, where Poland's past kings and national heroes are laid to rest.

The choice has sparked protests and a Facebook campaign by people opposed to Wawel as the place of burial of Kaczynski, a divisive figure in life due to his conservative, nationalist policies.

The row fractured the sense of national unity at the loss of the president and top figures, including Poland's four most senior military commanders and the governor of the central bank.

Black box voice recordings show that the crew from the Polish military jet knew they were doomed after hitting trees while trying to land, Polish authorities have said.

Russian investigators have said that pilot error was suspected in the plane crash near Smolensk, following the first analysis of the presidential jet's black box flight recorders.

They said the pilots may not have been aware that a manoeuvre they tried to carry out, levelling out the plane to compensate for thick fog, made the type of plane, a Russian-made Tupolev Tu-154, accelerate its descent.

Kaczynski's delegation had been headed to a ceremony marking the 70th anniversary of the Katyn massacre, when thousands of Polish officers were slaughtered by the Soviet secret police in 1940.